avformat/aviobuf: fix avio_flush() for read streams

avio_flush() did nothing useful for read streams. Fix it to behave as
expected, and discard the currently read buffer properly.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/utils: Force mutex to NULL after destruction.

A badly behaving user provided mutex manager (such as that in OpenCV) may not reset the mutex to NULL on destruction.  This can cause a problem for a later mutex manager (which may assert that the mutex is NULL before creating).

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