fix corner case in evaluate (#4120)

fixes #4119

lib/compress.js

@@ -3362,6 +3362,7 @@ merge(Compressor.prototype, {
             cached.forEach(function(node) {
                 delete node._eval;
             });
+            if (cached.unsafe) return this;
             if (ignore_side_effects) return val;
             if (!val || val instanceof RegExp) return val;
             if (typeof val == "function" || typeof val == "object") return this;
@@ -3430,6 +3431,10 @@ merge(Compressor.prototype, {
             var value = node._eval(compressor, ignore_side_effects, cached, depth);
             if (value === node) return this;
             modified(lhs);
+            if (Array.isArray(value)) value.toString = function() {
+                cached.unsafe = true;
+                return "[]";
+            };
             return value;
         });
         def(AST_Sequence, function(compressor, ignore_side_effects, cached, depth) {