folly: ubsan: reduce vector size to avoid UBSAN timeout

folly: ubsan: reduce vector size to avoid UBSAN timeout

Summary:
Based on diff where this was introduced in {D360195} it seems like

```
  // This value should we multiple of word size.
  static size_t const kHeapifyCapacitySize = sizeof(
    typename std::aligned_storage<
      sizeof(InternalSizeType),
      alignof(value_type)
    >::type);
  // Threshold to control capacity heapifying.
  static size_t const kHeapifyCapacityThreshold =
    100 * kHeapifyCapacitySize;
```

So anything above 100*sizeof(SizeType) should do.

Reviewed By: philippv

Differential Revision: D2871422

fb-gh-sync-id: a69e47286c53887ac05e89dab565b9d609e183a0