pjsip/dialplan_functions: Handle PJSIP_MEDIA_OFFER called on non-PJSIP channels

Calling PJSIP_MEDIA_OFFER on a non-PJSIP channel is hazardous to your health.
It will treat the channels as a PJSIP channel, eventually hitting an ao2 error,
FRACKing on assertion error, and quite likely crashing.

This patch adds checks to the read/write callbacks that ensure that the channel
technology is of type 'PJSIP' before attempting to operate on the channel.

#SIPit31

ASTERISK-24382 #close
Reported by: Matt Jordan
........

Merged revisions 424621 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@424622 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/pjsip/dialplan_functions.c

@@ -707,7 +707,7 @@ int pjsip_acf_channel_read(struct ast_channel *chan, const char *cmd, char *data
 
 	/* Sanity check */
 	if (strcmp(ast_channel_tech(chan)->type, "PJSIP")) {
-		ast_log(LOG_ERROR, "Cannot call %s on a non-PJSIP channel\n", cmd);
+		ast_log(LOG_WARNING, "Cannot call %s on a non-PJSIP channel\n", cmd);
 		return 0;
 	}
 
@@ -866,6 +866,11 @@ int pjsip_acf_media_offer_read(struct ast_channel *chan, const char *cmd, char *
 		return -1;
 	}
 
+	if (strcmp(ast_channel_tech(chan)->type, "PJSIP")) {
+		ast_log(LOG_WARNING, "Cannot call %s on a non-PJSIP channel\n", cmd);
+		return -1;
+	}
+
 	channel = ast_channel_tech_pvt(chan);
 
 	if (!strcmp(data, "audio")) {
@@ -889,6 +894,11 @@ int pjsip_acf_media_offer_write(struct ast_channel *chan, const char *cmd, char
 		return -1;
 	}
 
+	if (strcmp(ast_channel_tech(chan)->type, "PJSIP")) {
+		ast_log(LOG_WARNING, "Cannot call %s on a non-PJSIP channel\n", cmd);
+		return -1;
+	}
+
 	channel = ast_channel_tech_pvt(chan);
 	mdata.session = channel->session;