Code:  static uint64_t ReadBits(BitReader* reader, int num_bits) {
   DCHECK_GE(reader->bits_available(), num_bits);
   DCHECK((num_bits > 0) && (num_bits <= 64));
  uint64_t value;
  reader->ReadBits(num_bits, &value);
   return value;
 }

Commit Message: 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}
CWE ID: CWE-200
 static uint64_t ReadBits(BitReader* reader, int num_bits) {
   DCHECK_GE(reader->bits_available(), num_bits);
   DCHECK((num_bits > 0) && (num_bits <= 64));
  uint64_t value = 0;

  if (!reader->ReadBits(num_bits, &value))
    return 0;

   return value;
 }
