BitUnStuff(): avoid other unsigned integer overflow
BitUnStuff(): avoid other unsigned integer overflow

- There was a missing parenthesis when computing numUInts which resulted
  in the cast to unsigned int being done before the division by 32, and not
  after.
- If numElements is big enough and numBits as well, then numUInts can be
  close to UINT32_MAX, leading to numBytes potentially overflowing. So
  use size_t for numBytes, and for 32-bit builds, add a check for overflow.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=16714

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