bug#15051: 24.3; gdb -mi: breakpoint with in non-existent source file spams terminal incessantly

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Sun, 06 Dec 2020 16:41:16 +0100
> Cc: 15051@debbugs.gnu.org
> 
> Thanks for the clear recipe to reproduce this bug.  It is still present
> in Emacs 28, but has changed, and is now even worse, I think.
> 
> First of all:
> 
> >
> > 6. Execute the following commands in gdb:
> > b bar
> > start
> > n
> > n
> > fin
> > n
> > n
> > n
> > quit
> 
> After "b bar", Emacs says:
> 
> File /tmp/"/tmp/bar.c" is read-only on disk.  Make buffer read-only, too? (y or n) n
> 
> Then, it's these, as you point out:
> 
> (gdb) b bar
> Breakpoint 1 at 0x117e: file bar.c, line 1.
> (gdb) list bar.c:1
> 1	bar.c: No such file or directory.
> start
> Temporary breakpoint 2 at 0x1139: file foo.c, line 4.
> Starting program: /tmp/bug 
> 
> Temporary breakpoint 2, main () at foo.c:4
> 4	puts("Thinking about calling bar");;
> (gdb) list bar.c:1
> 1	bar.c: No such file or directory.
> n
> 5	puts("Planning to call bar");
> (gdb) list bar.c:1
> 1	bar.c: No such file or directory.
> n
> 6	puts("About to call bar");
> (gdb) list bar.c:1
> 1	bar.c: No such file or directory.
> n
> 7	bar();
> 
> And then I got 20K copies of:
> 
> list bar.c:1
> 1	in bar.c
> list bar.c:1
> 1	in bar.c
> list bar.c:1
> 1	in bar.c
> list bar.c:1
> 1	in bar.c
> 
> which continued until I killed gdb.
> 
> So this problem is still present in Emacs 28, but is now somewhat
> different.

I've now fixed this on the master branch.