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}
static bool StartsWith(const uint8_t* buffer,
                       size_t buffer_size,
                       const char* prefix) {
  size_t prefix_size = strlen(prefix);
  return (prefix_size <= buffer_size &&
          memcmp(buffer, prefix, prefix_size) == 0);
}

static bool StartsWith(const uint8_t* buffer,
                       size_t buffer_size,
                       const char* prefix) {
  size_t prefix_size = strlen(prefix);
  return (prefix_size <= buffer_size &&
          memcmp(buffer, prefix, prefix_size) == 0);
}
