avformat/mpegts: Warn if ffio_ensure_seekback() failed

Should silence CID1257005

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/buffer: add #if USE_ATOMICS around atomics specific functions to ensure sync types arent mixed by mistake

Fixes CID1257011

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