fix corner case in join_vars (#3857)

fixes #3856

lib/compress.js

@@ -2579,6 +2579,8 @@ merge(Compressor.prototype, {
                     }
                     if (node instanceof AST_Scope) return node;
                     if (!(node instanceof AST_Statement)) return node;
+                }, function(node) {
+                    if (node instanceof AST_For && is_empty(node.init)) node.init = null;
                 }));
             }
         }