Fix crash in ada_print_type

The "varstring" paramter to ada_print_type can be null, but one spot
failed to check this.  This could cause a crash in some situations.

As this is Ada-specific, and we've been using it internally at AdaCore
for a while, I am going to push it.

gdb/ada-typeprint.c

@@ -925,8 +925,8 @@ print_func_type (struct type *type, struct ui_file *stream, const char *name,
 
 /* Print a description of a type TYPE0.
    Output goes to STREAM (via stdio).
-   If VARSTRING is a non-empty string, print as an Ada variable/field
+   If VARSTRING is a non-NULL, non-empty string, print as an Ada
-       declaration.
+       variable/field declaration.
    SHOW+1 is the maximum number of levels of internal type structure
       to show (this applies to record types, enumerated types, and
       array types).
@@ -996,7 +996,8 @@ ada_print_type (struct type *type0, const char *varstring,
 	/* An __XVL field is not truly a pointer, so don't print
 	   "access" in this case.  */
 	if (type->code () != TYPE_CODE_PTR
-	    || strstr (varstring, "___XVL") == nullptr)
+	    || (varstring != nullptr
+		&& strstr (varstring, "___XVL") == nullptr))
 	  gdb_printf (stream, "access ");
 	ada_print_type (type->target_type (), "", stream, show, level,
 			flags);

gdb/testsuite/gdb.ada/overload_menu_crash.exp

@@ -0,0 +1,46 @@
+# Copyright 2022 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Regression test for a crash in the overload menu.
+
+load_lib "ada.exp"
+
+if { [skip_ada_tests] } { return -1 }
+
+standard_ada_testfile main
+
+if {[gdb_compile_ada "$srcfile" "$binfile" executable {debug}] != ""} {
+    return -1
+}
+
+clean_restart ${testfile}
+
+set bp_location [gdb_get_line_number "BREAK" ${testdir}/main.adb]
+runto "main.adb:$bp_location"
+
+set menu [multi_line "Multiple matches for regtest" \
+	      "\\\[0\\\] cancel" \
+	      "\\\[1\\\] pck.inner.regtest at .*pck.ads:\[0-9\]+" \
+	      "\\\[2\\\] pck.regtest .* return boolean at .*pck.adb:\[0-9\]+" \
+	      "> $"]
+gdb_test_multiple "whatis &regtest" "menu does not crash" {
+    -re "$menu" {
+	pass "$gdb_test_name"
+    }
+    default {
+	fail "$gdb_test_name"
+    }
+}
+gdb_test "1" "type = access boolean" "choose from menu"

gdb/testsuite/gdb.ada/overload_menu_crash/main.adb

@@ -0,0 +1,21 @@
+--  Copyright 2022 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+with Pck;
+
+procedure Main is
+begin
+   Pck.Inner.Regtest := Pck.Regtest (null); -- BREAK
+end Main;

gdb/testsuite/gdb.ada/overload_menu_crash/pck.adb

@@ -0,0 +1,21 @@
+--  Copyright 2022 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package body Pck is
+   function Regtest (A : access Integer) return Boolean is
+   begin
+      return True;
+   end Regtest;
+end Pck;

gdb/testsuite/gdb.ada/overload_menu_crash/pck.ads

@@ -0,0 +1,22 @@
+--  Copyright 2022 Free Software Foundation, Inc.
+--
+--  This program is free software; you can redistribute it and/or modify
+--  it under the terms of the GNU General Public License as published by
+--  the Free Software Foundation; either version 3 of the License, or
+--  (at your option) any later version.
+--
+--  This program is distributed in the hope that it will be useful,
+--  but WITHOUT ANY WARRANTY; without even the implied warranty of
+--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+--  GNU General Public License for more details.
+--
+--  You should have received a copy of the GNU General Public License
+--  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package Pck is
+   package Inner is
+      Regtest : Boolean;
+   end Inner;
+
+   function Regtest (A : access Integer) return Boolean;
+end Pck;