Prevent crash when an SDP offer is received with an encrypted video stream...

Prevent crash when an SDP offer is received with an encrypted video stream when support for video is disabled and res_srtp is loaded.

(closes issue ASTERISK-19202)
Reported by: Catalin Sanda
........

Merged revisions 351504 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 351505 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@351506 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -30275,6 +30275,12 @@ static int setup_srtp(struct sip_srtp **srtp)
 static int process_crypto(struct sip_pvt *p, struct ast_rtp_instance *rtp, struct sip_srtp **srtp, const char *a)
 {
+	/* If no RTP instance exists for this media stream don't bother processing the crypto line */
+	if (!rtp) {
+		ast_debug(3, "Received offer with crypto line for media stream that is not enabled\n");
+		return FALSE;
+	}
+
 	if (strncasecmp(a, "crypto:", 7)) {
 		return FALSE;
 	}