Container object's internal object must be allocated firstly (#2961)

This patch slightly reworks the container objects internal objects allocation.
This rework allows the same lifetime of the objects without the manual allocation/deallocation of the internal object.

This patch also fixes #2951.

Co-authored-by: Dániel Bátyai <dbatyai@inf.u-szeged.hu>
JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

tests/jerry/es2015/regression-test-issue-2951.js

@@ -0,0 +1,16 @@
+// 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.
+
+// To trigger the assertion the engine must be compiled with --system allocator=ON and --mem-stress-test=ON
+m = new Map([]);