avcodec/pthread_frame: clear priv_data, avoid stale pointer in error case

Fixes: b4b47bc2b3fb7ca710bfffe5aa969e37_signal_sigabrt_7ffff70eccc9_744_nc_sample2.avi with memlimit of 4194304

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>