gpg-interface: do not scan past the end of buffer

If the GPG output ends with trailing blank lines, after skipping
them over inside the loop to find the terminating NUL at the end,
the loop ends up looking for the next line, starting past the end.

Signed-off-by: Steven Roberts <sroberts@fenderq.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -116,6 +116,9 @@ static void parse_gpg_output(struct signature_check *sigc)
 	for (line = buf; *line; line = strchrnul(line+1, '\n')) {
 		while (*line == '\n')
 			line++;
+		if (!*line)
+			break;
+
 		/* Skip lines that don't start with GNUPG status */
 		if (!skip_prefix(line, "[GNUPG:] ", &line))
 			continue;