Address a clang-analyzer warning about undefined behavior in filter.
Address a clang-analyzer warning about undefined behavior in filter.

- Analyzer didn't see that code is never called with MAX_DEPTH as it doesn't
  know the properties of width, height, x and y. Following would also
  silence the error:
    assert(cur_pic->width > 0 && cur_pic->height > 0);
    assert(cur_pic->width % 8 == 8 && cur_pic->height % 8 == 0);
    assert(x > 0 && y > 0);
    assert(x % 8 == 0 && y % 8 == 0);

- Related to issue #35.

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