[gdb/tui] Fix breakpoint display functionality

In commit 81e6b8eb20 "Make tui-winsource not use breakpoint_chain", a loop
body was transformed into a lambda function body:
...
-      for (bp = breakpoint_chain;
-           bp != NULL;
-           bp = bp->next)
+      iterate_over_breakpoints ([&] (breakpoint *bp) -> bool
...
and consequently:
- a continue was replaced by a return, and
- a final return was added.

Then in commit 240edef62f "gdb: remove iterate_over_breakpoints function", we
transformed back to a loop body:
...
-      iterate_over_breakpoints ([&] (breakpoint *bp) -> bool
+      for (breakpoint *bp : all_breakpoints ())
...
but without reverting the changes that introduced the two returns.

Consequently, breakpoints no longer show up in the tui source window.

Fix this by reverting the changes that introduced the two returns.

Build on x86_64-linux, tested with all .exp test-cases that contain
tuiterm_env.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28483

gdb/testsuite/gdb.tui/break.exp

@@ -0,0 +1,37 @@
+# Copyright 2021 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/>.
+
+# Verify that breakpoint marker is shown.
+
+tuiterm_env
+
+standard_testfile tui-layout.c
+
+if {[build_executable "failed to prepare" ${testfile} ${srcfile}] == -1} {
+    return -1
+}
+
+Term::clean_restart 24 80 $testfile
+if {![Term::enter_tui]} {
+    unsupported "TUI not supported"
+    return
+}
+
+set text [Term::get_all_lines]
+gdb_assert {![string match "No Source Available" $text]} \
+    "initial source listing"
+
+Term::command "b main"
+Term::check_contents "break marker present" "\\|b\\+"

gdb/tui/tui-winsource.c

@@ -460,7 +460,7 @@ tui_source_window_base::update_breakpoint_info
       for (breakpoint *bp : all_breakpoints ())
 	{
 	  if (bp == being_deleted)
-	    return false;
+	    continue;
 
 	  for (bp_location *loc : bp->locations ())
 	    {
@@ -478,7 +478,6 @@ tui_source_window_base::update_breakpoint_info
 		    mode |= TUI_BP_HARDWARE;
 		}
 	    }
-	  return false;
 	}
 
       if (line->break_mode != mode)