[gdb/tui] Fix breakpoint display functionality

[gdb/tui] Fix breakpoint display functionality

In commit 81e6b8eb208 "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 240edef62f0 "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