Evaluate [...].join() if possible: minor bugfix

Follow-up to 78e98d2.

lib/compress.js

@@ -655,6 +655,8 @@ merge(Compressor.prototype, {
             throw def;
         });
         function ev(node, compressor) {
+            if (!compressor) throw new Error("Compressor must be passed");
+
             return node._eval(compressor);
         };
         def(AST_Node, function(){
@@ -700,9 +702,6 @@ merge(Compressor.prototype, {
               case "+"          :
                 // handle concatenating strings even if the left part cannot
                 // be evaluated, e.g. (variable + "str") + "str"
-                if (!(c instanceof Compressor)) {
-                    throw new Error("Compressor must be passed!!");
-                }
                 if (left instanceof AST_Binary && left.operator == "+" && left.is_string(c)) {
                     return make_node(AST_Binary, this, {
                         operator: "+",
@@ -754,7 +753,7 @@ merge(Compressor.prototype, {
                     var x =  this.expression.expression.elements.map(function(el){
                         return ev(el, compressor);
                     });
-                    return x.join(ev(this.args[0]));
+                    return x.join(ev(this.args[0], compressor));
                 }
             }
             throw def;