avutil/buffer: Fix race in pool.

This race will always happen sooner or later in a multi-threaded
environment and it will over time lead to OOM.
This fix works by spinning, there are other ways by which this
can be fixed, like simply detecting the issue after it happened
and freeing the over-allocated memory or simply using a mutex.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>