gpg-interface: handle missing " with " gracefully in parse_ssh_output()

gpg-interface: handle missing " with " gracefully in parse_ssh_output()

If the output of ssh-keygen starts with "Good \"git\" signature for ",
but is not followed by " with " for some reason, then parse_ssh_output()
uses -1 as the len parameter of xmemdupz(), which in turn will end the
program.  Reject the signature and carry on instead in that case.

Signed-off-by: René Scharfe <redacted>
Acked-by: Fabian Stelzer <redacted>
Signed-off-by: Junio C Hamano <redacted>