gpg-interface: avoid buffer overrun in parse_ssh_output()

gpg-interface: avoid buffer overrun in parse_ssh_output()

If the string "key" we found in the output of ssh-keygen happens to be
located at the very end of the line, then going four characters further
leaves us beyond the end of the string.  Explicitly search for the
space after "key" to handle a missing one gracefully.

Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>