[idl_parser] Validate force_align on all possible paths (#6430)

* [idl_parser] Validate `force_align` on all possible paths

- added validation of `force_align` for `Vector`.
- added assertion to `flatbuffers::PreAlign` method

These changes should resolve following oss-fuzz issues:
- 6275816289861632
- 5713529908887552
- 4761242994606080

* size_t problem on Mac

* Fix review notes

include/flatbuffers/util.h

@@ -335,6 +335,9 @@ inline bool StringToFloatImpl(T *val, const char *const str) {
 // - If the converted value falls out of range of corresponding return type, a
 // range error occurs. In this case value MAX(T)/MIN(T) is returned.
 template<typename T> inline bool StringToNumber(const char *s, T *val) {
+  // Assert on `unsigned long` and `signed long` on LP64.
+  // If it is necessary, it could be solved with flatbuffers::enable_if<B,T>.
+  static_assert(sizeof(T) < sizeof(int64_t), "unexpected type T");
   FLATBUFFERS_ASSERT(s && val);
   int64_t i64;
   // The errno check isn't needed, will return MAX/MIN on overflow.