Cleanup media BitReader ReadBits() calls

Initialize temporary values, check return values.
Small tweaks to solution proposed by adtolbar@microsoft.com.

Bug: 929962
Change-Id: Iaa7da7534174882d040ec7e4c353ba5cd0da5735
Reviewed-on: https://chromium-review.googlesource.com/c/1481085
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634889}
bool BitReaderCore::SkipBitsSmall(int num_bits) {
  DCHECK_GE(num_bits, 0);
  uint64_t dummy;
  while (num_bits >= kRegWidthInBits) {
    if (!ReadBitsInternal(kRegWidthInBits, &dummy))
      return false;
    num_bits -= kRegWidthInBits;
  }
  return ReadBitsInternal(num_bits, &dummy);
}

bool BitReaderCore::SkipBitsSmall(int num_bits) {
  DCHECK_GE(num_bits, 0);
  uint64_t dummy;
  while (num_bits >= kRegWidthInBits) {
    if (!ReadBitsInternal(kRegWidthInBits, &dummy))
      return false;
    num_bits -= kRegWidthInBits;
  }
  return ReadBitsInternal(num_bits, &dummy);
}
