chan_sip.c: Fix crash processing CANCEL.

Check if initreq data string exists before using it when processing a
CANCEL request.

ASTERISK-27666

Change-Id: Id1d0f0fa4ec94e81b332b2973d93e5a14bb4cc97

channels/chan_sip.c

@@ -27264,7 +27264,7 @@ static int handle_request_cancel(struct sip_pvt *p, struct sip_request *req)
 	} else {
 		sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
 	}
-	if (ast_str_strlen(p->initreq.data) > 0) {
+	if (p->initreq.data && ast_str_strlen(p->initreq.data) > 0) {
 		struct sip_pkt *pkt, *prev_pkt;
 		/* If the CANCEL we are receiving is a retransmission, and we already have scheduled
 		 * a reliable 487, then we don't want to schedule another one on top of the previous