Schema parser: prohibit declaration of an array of pointers inside structs (#5907)

* Fix issue #5906, Prohibit declaration of an array of pointers inside structs

- idl_parser.cpp: Prohibit declaration of an array of pointers inside structs
- idl_gen_cpp.cpp: Extract GenStructConstructor() method from GenStruct() to simplify future modification
- idl_gen_cpp.cpp: Add assert for checking of Array fields in structs on code-generation stage

* Fix the error 'unused local variable' in release build

* Fix: format the PR code according to coding rules

* Add test-case and fix review notes

tests/test.cpp

@@ -1623,6 +1623,9 @@ void ErrorTest() {
   TestError("table X { Y:int; } root_type X; { Y:1.0 }", "float");
   TestError("table X { Y:bool; } root_type X; { Y:1.0 }", "float");
   TestError("enum X:bool { Y = true }", "must be integral");
+  // Array of non-scalar
+  TestError("table X { x:int; } struct Y { y:[X:2]; }",
+            "may contain only scalar or struct fields");
 }
 
 template<typename T>