fix unsafe join() on array literal with spread (#2347)

fixes #2345

lib/compress.js

@@ -3437,7 +3437,9 @@ merge(Compressor.prototype, {
                 }
                 var elements = [];
                 var consts = [];
-                exp.expression.elements.forEach(function(el) {
+                for (var i = 0, len = exp.expression.elements.length; i < len; i++) {
+                    var el = exp.expression.elements[i];
+                    if (el instanceof AST_Expansion) break EXIT;
                     var value = el.evaluate(compressor);
                     if (value !== el) {
                         consts.push(value);
@@ -3450,7 +3452,7 @@ merge(Compressor.prototype, {
                         }
                         elements.push(el);
                     }
-                });
+                }
                 if (consts.length > 0) {
                     elements.push(make_node(AST_String, self, {
                         value: consts.join(separator)