bug#34769: Bug in emacs 26.1 gdb-send match-string applied after non-matching string-match

[Tobias Zawada]

Dear Emacs maintainers,

At the end of gdb-send one finds the following lines:

  (let* ((control-command-p (string-match gdb-control-commands-regexp string))
         (command-arg (match-string 3 string))

It is wrong to call (match-string 3 string) if (string-match 
gdb-control-commands-regexp string) returned nil.
The doc of match-string says:
"Return string of text matched by last search."
That means match-string only returns sensible results if the last match was 
successful.

Possible correction:

  (let* ((control-command-p (string-match gdb-control-commands-regexp string))
         (command-arg (and control-command-p (match-string 3 string)))


System info:
GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.30) of 
2018-05-29

Best regards,
Tobias