fix destructuring bugs in mangle and compress (#1992)

- destructuring mangle
- destructuring array default values

fixes #1335

lib/compress.js

@@ -2097,9 +2097,6 @@ merge(Compressor.prototype, {
                                 }));
                             }
                         }
-                        else if (node.names[i] instanceof AST_Hole) {
-                            continue;
-                        }
                         else if (node.names[i] instanceof AST_ObjectKeyVal) {
                             if (typeof node.names[i].key === "string") {
                                 initializations.add(node.names[i].key, destructuring_value);
@@ -2107,7 +2104,14 @@ merge(Compressor.prototype, {
                         }
                         else if (node.names[i] instanceof AST_Symbol) {
                             initializations.add(node.names[i].name, destructuring_value);
-                        } else {
+                        }
+                        else if (node.names[i] instanceof AST_DefaultAssign) {
+                            continue;
+                        }
+                        else if (node.names[i] instanceof AST_Hole) {
+                            continue;
+                        }
+                        else {
                             throw new Error(string_template("Unknown destructuring element of type: {type}", {
                                 type: Object.getPrototypeOf(node.names[i]).TYPE
                             }));