avio: Do not consider the end-of-buffer position valid

Trigger a refill if the seek action moves the pointer
at the end of the buffer.

Before this patch the read action following the seek would trigger
the refill, while write action would write outside the buffer.

In the Libav codebase few muxers seek forward outside of what
already has been written so it is quite unlikely to experience
the problem with the default buffer size.

CC: libav-stable@libav.org
