[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/idl.h

@@ -967,6 +967,8 @@ class Parser : public ParserState {
   FLATBUFFERS_CHECKED_ERROR CheckClash(std::vector<FieldDef *> &fields,
                                        StructDef *struct_def,
                                        const char *suffix, BaseType baseType);
+  FLATBUFFERS_CHECKED_ERROR ParseAlignAttribute(
+      const std::string &align_constant, size_t min_align, size_t *align);
 
   bool SupportsAdvancedUnionFeatures() const;
   bool SupportsAdvancedArrayFeatures() const;