jmls pointed out that unsetting the group and setting the group to the blank string aren't quite the same.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@83773 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/app.c

@@ -815,7 +815,9 @@ int ast_app_group_set_channel(struct ast_channel *chan, const char *data)
 	}
 	AST_LIST_TRAVERSE_SAFE_END
 
-	if ((gi = calloc(1, len))) {
+	if (ast_strlen_zero(group)) {
+		/* Enable unsetting the group */
+	} else if ((gi = calloc(1, len))) {
 		gi->chan = chan;
 		gi->group = (char *) gi + sizeof(*gi);
 		strcpy(gi->group, group);