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>

libavcodec/pthread_frame.c

@@ -690,6 +690,7 @@ int ff_frame_thread_init(AVCodecContext *avctx)
 
         copy->internal = av_malloc(sizeof(AVCodecInternal));
         if (!copy->internal) {
+            copy->priv_data = NULL;
             err = AVERROR(ENOMEM);
             goto error;
         }