Avoid crash on event without content-type

If we received an event without a content-type header we were
dereferencing a null pointer leading to a seg fault.
Reported-by: Ico <ico@voip-io.org>

ESL-90 --resolve

libs/esl/src/esl.c

@@ -1556,7 +1556,7 @@ ESL_DECLARE(esl_status_t) esl_send_recv_timed(esl_handle_t *handle, const char *
 	if (handle->last_sr_event) {
 		char *ct = esl_event_get_header(handle->last_sr_event,"content-type");
 
-		if (strcasecmp(ct, "api/response") && strcasecmp(ct, "command/reply")) {
+		if (ct && strcasecmp(ct, "api/response") && strcasecmp(ct, "command/reply")) {
 			esl_event_t *ep;
 
 			for(ep = handle->race_event; ep && ep->next; ep = ep->next);