bug#61271: 30.0.50; gud makes source files un-editable

[Dima Kogan <dkogan@debian.org>]

Hi. I'm using a bleeding edge build of emacs from git. This issue is
recent: probably a few weeks old at most. Recipe:

1. Create any debuggable C program. For instance I just made a
   tst.c:

     int main(int argc, char* argv[])
     {
         return 0;
     }

   And I built it like this:

     gcc -g -o tst tst.c

2. emacs -Q tst.c

3. M-x gud-gdb ... gdb --fullname tst

4. b main

5. r

   We're now debugging the executable tst, and we're at a breakpoint in
   tst.c. emacs should show the breakpoint in the tst.c buffer

6. Switch to the tst.c buffer

7. Press RET

tst.c is a source buffer. "RET" should insert a newline. Instead emacs
throws an error

  comint-send-input: Current buffer has no process

Pressing f1-b I see that tst.c is in gud-minor-mode (although this
doesn't show up in f1-m for some reason). This mode defines the RET
binding, and is the source of this problem.

Thanks