res_pjsip_sdp_rtp: Restore removed NULL check.

When sending an RTP keepalive, we need to be sure we're not dealing with
a NULL RTP instance. There had been a NULL check, but the commit that
added the rtp_timeout and rtp_hold_timeout options removed the NULL
check.

Change-Id: I2d7dcd5022697cfc6bf3d9e19245419078e79b64

res/res_pjsip_sdp_rtp.c

@@ -115,6 +115,10 @@ static int send_keepalive(const void *data)
 	time_t interval;
 	int send_keepalive;
 
+	if (!rtp) {
+		return 0;
+	}
+
 	keepalive = ast_rtp_instance_get_keepalive(rtp);
 
 	if (!ast_sockaddr_isnull(&session_media->direct_media_addr)) {