Fix built-in RegExp allocation with default constructor (#3883)

Fixes #3880.
Fixes #3881.

JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

tests/jerry/es2015/regression-test-issue-3880.js

@@ -0,0 +1,21 @@
+// Copyright JS Foundation and other contributors, http://js.foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+RegExp.prototype.constructor = ""
+var r = /a/;
+assert (r.test ("a"));
+
+RegExp.prototype.constructor = {}
+r = /b/;
+assert (r.test ("b"));