avutil: add FF_DECODE_ERROR_CONCEALMENT_ACTIVE flag for AVFrame.decode_error_flags

FF_DECODE_ERROR_CONCEALMENT_ACTIVE is set when the decoded frame has error(s) but the returned value from
avcodec_receive_frame is zero i.e. concealed errors

Signed-off-by: Amir Pauker <amir@livelyvideo.tv>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>