avformat/aviobuf: fix checks in ffio_ensure_seekback

avformat/aviobuf: fix checks in ffio_ensure_seekback

The new buf_size was detemined too conservatively, maybe because of the
off-by-one issue which was fixed recently in fill_buffer. We can safely
substract 1 more from the new buffer size, because max_buffer_size space must
only be guaranteed when we are reading the last byte of the requested window.

Comparing the new buf_size against filled did not make a lot of sense, what
makes sense is that we want to reallocate the buffer if the new buf_size is
bigger than the old, therefore the change in the check.

Signed-off-by: Marton Balint <cus@passwd.hu>