[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/flatbuffers.h

@@ -1670,11 +1670,13 @@ class FlatBufferBuilder {
   // This is useful when storing a nested_flatbuffer in a vector of bytes,
   // or when storing SIMD floats, etc.
   void ForceVectorAlignment(size_t len, size_t elemsize, size_t alignment) {
+    FLATBUFFERS_ASSERT(VerifyAlignmentRequirements(alignment));
     PreAlign(len * elemsize, alignment);
   }
 
   // Similar to ForceVectorAlignment but for String fields.
   void ForceStringAlignment(size_t len, size_t alignment) {
+    FLATBUFFERS_ASSERT(VerifyAlignmentRequirements(alignment));
     PreAlign((len + 1) * sizeof(char), alignment);
   }