fix mem leak in free_value (bug #2990)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@4398 65c4cc65-6c06-0410-ace0-fbb531ad65f3

ast_expr.y

@@ -215,6 +215,8 @@ struct val *vp;
 	}
 	if (vp->type == string || vp->type == numeric_string)
 		free (vp->u.s);	
+	if (vp)
+		free (vp);
 }