don't blow up if a jitterbuffer is not in use

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@35555 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channel.c

@@ -1015,7 +1015,8 @@ void ast_channel_free(struct ast_channel *chan)
 		ast_var_delete(vardata);
 
 	/* Destroy the jitterbuffer */
-	ast_jb_destroy(chan);
+	if (chan->jb)
+		ast_jb_destroy(chan);
 
 	ast_string_field_free_all(chan);
 	free(chan);