Add a log message for when DTLS-SRTP is requested and the underlying engine does not support it.

(closes issue ASTERISK-20487)
Reported by: mjordan


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@374755 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -5534,6 +5534,8 @@ static int dialog_initialize_dtls_srtp(const struct sip_pvt *dialog, struct ast_
 	if (!(dtls = ast_rtp_instance_get_dtls(rtp)) ||
 	    dtls->set_configuration(rtp, &dialog->dtls_cfg) ||
 	    !(*srtp = sip_srtp_alloc())) {
+		ast_log(LOG_ERROR, "No DTLS-SRTP support present on engine for RTP instance '%p', was it compiled with support for it?\n",
+			instance);
 		return -1;
 	}