gdb: fix potentially uninitialised variable

In commit:

  commit 037d7135de
  Date:   Mon Nov 16 11:36:56 2020 +0000

      gdb: improve command completion for 'print', 'x', and 'display'

A potential use of an uninitialised variable was introduced.  This is
fixed in this commit.

Previously when analysing /FMT strings for tab completion we
considered two possibilities, either the user has typed '/', or the
user has typed '/' followed by an alpha-numeric character, as these
are the only valid FMT string characters.

This meant that if the user type, for example '/@' and then tried to
tab complete gdb would use an uninitialised variable.

Currently only the first character after the '/' is checked to see if
it is alpha-numeric, so if a user typed '/x@@' then gdb would be happy
to treat this as a FMT string.

Given the goal of this change was primarily to allow tab completion of
symbols later in the command when a /FMT was used then I decided to
just make the /FMT skipping less smart.  Now any characters after the
'/' up to the first white space, will be treated as a FMT string.

gdb/ChangeLog:

	* printcmd.c (skip_over_slash_fmt): Reorder code to ensure in_fmt
	is always initialized.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-11-27  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* printcmd.c (skip_over_slash_fmt): Reorder code to ensure in_fmt
+	is always initialized.
+
 2020-11-26  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>
 
 	* features/aarch64-fpu.c (create_feature_aarch64_fpu): Regenerate.

gdb/printcmd.c

@@ -1258,27 +1258,38 @@ skip_over_slash_fmt (completion_tracker &tracker, const char **args)
       bool in_fmt;
       tracker.set_use_custom_word_point (true);
 
-      if (ISALNUM (text[1]) || ISSPACE (text[1]))
+      if (text[1] == '\0')
 	{
-	  /* Skip over the actual format specification.  */
+	  /* The user tried to complete after typing just the '/' character
+	     of the /FMT string.  Step the completer past the '/', but we
+	     don't offer any completions.  */
+	  in_fmt = true;
+	  ++text;
+	}
+      else
+	{
+	  /* The user has typed some characters after the '/', we assume
+	     this is a complete /FMT string, first skip over it.  */
 	  text = skip_to_space (text);
 
 	  if (*text == '\0')
 	    {
+	      /* We're at the end of the input string.  The user has typed
+		 '/FMT' and asked for a completion.  Push an empty
+		 completion string, this will cause readline to insert a
+		 space so the user now has '/FMT '.  */
 	      in_fmt = true;
 	      tracker.add_completion (make_unique_xstrdup (text));
 	    }
 	  else
 	    {
+	      /* The user has already typed things after the /FMT, skip the
+		 whitespace and return false.  Whoever called this function
+		 should then try to complete what comes next.  */
 	      in_fmt = false;
 	      text = skip_spaces (text);
 	    }
 	}
-      else if (text[1] == '\0')
-	{
-	  in_fmt = true;
-	  ++text;
-	}
 
       tracker.advance_custom_word_point_by (text - *args);
       *args = text;