Fix segfault when dealing with chan_agent channels.

Check the returned bridged pointer for NULL to avoid a crash.  It looks
like chan_agent is returning a NULL pointer when it probably should be
returning a pointer to the channel the Agent channel is pretending to be.

(closes issue ASTERISK-21793)
Reported by: Rodrigo P. Telles
Patches:
      jira_asterisk_21793_v1.8.patch (license #5621) patch uploaded by rmudgett
Tested by: Rodrigo P. Telles
........

Merged revisions 390044 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 390047 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390068 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/channel.c

@@ -6638,14 +6638,14 @@ void ast_channel_set_linkgroup(struct ast_channel *chan, struct ast_channel *pee
 	linkedid = oldest_linkedid(linkedid, ast_channel_uniqueid(peer));
 	if (ast_channel_internal_bridged_channel(chan)) {
 		bridged = ast_bridged_channel(chan);
-		if (bridged != peer) {
+		if (bridged && bridged != peer) {
 			linkedid = oldest_linkedid(linkedid, ast_channel_linkedid(bridged));
 			linkedid = oldest_linkedid(linkedid, ast_channel_uniqueid(bridged));
 		}
 	}
 	if (ast_channel_internal_bridged_channel(peer)) {
 		bridged = ast_bridged_channel(peer);
-		if (bridged != chan) {
+		if (bridged && bridged != chan) {
 			linkedid = oldest_linkedid(linkedid, ast_channel_linkedid(bridged));
 			linkedid = oldest_linkedid(linkedid, ast_channel_uniqueid(bridged));
 		}
@@ -6658,13 +6658,13 @@ void ast_channel_set_linkgroup(struct ast_channel *chan, struct ast_channel *pee
 	ast_channel_change_linkedid(peer, linkedid);
 	if (ast_channel_internal_bridged_channel(chan)) {
 		bridged = ast_bridged_channel(chan);
-		if (bridged != peer) {
+		if (bridged && bridged != peer) {
 			ast_channel_change_linkedid(bridged, linkedid);
 		}
 	}
 	if (ast_channel_internal_bridged_channel(peer)) {
 		bridged = ast_bridged_channel(peer);
-		if (bridged != chan) {
+		if (bridged && bridged != chan) {
 			ast_channel_change_linkedid(bridged, linkedid);
 		}
 	}