Fix crash on inter frames.
Fix crash on inter frames.

- If the bitcost was 0 it would underflow for skip mode. The bitcost is now
  checked before decrementing.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,lu-zero/kvazaar,ultravideo/kvazaar,lu-zero/kvazaar,lu-zero/kvazaar,lu-zero/kvazaar