Poisoning returned buffers is based around the implicit assumption
that the contents of said buffers are transient. Yet this is not true
for the buffer pools used by the various hardware contexts which store
important state in there that needs to be preserved.
Furthermore, the current code is also based on the assumption
that the complete buffer pointed to by AVBuffer->data coincides with
AVBufferRef->data; yet an implementation might store some data of its
own before the actual user-visible data (accessible via AVBufferRef)
which would be broken by the current code.

(This is of course yet more proof that the AVBuffer API is not the right
tool for the hardware contexts.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>