imap-send: handle missing response codes gracefully

Response codes are optional.  Exit parse_response_code() early if it's
passed a NULL string, indicating that we reached the end of the reply.
This avoids dereferencing said NULL pointer.

Noticed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -683,7 +683,7 @@ static int parse_response_code(struct imap_store *ctx, struct imap_cmd_cb *cb,
 	struct imap *imap = ctx->imap;
 	char *arg, *p;
 
-	if (*s != '[')
+	if (!s || *s != '[')
 		return RESP_OK;		/* no response code */
 	s++;
 	if (!(p = strchr(s, ']'))) {