avcodec/options: leave priv_data in avcodec_copy_context() intact

previously it was freed but the pointer not cleared

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

libavcodec/options.c

@@ -182,7 +182,6 @@ int avcodec_copy_context(AVCodecContext *dest, const AVCodecContext *src)
     }
 
     av_opt_free(dest);
-    av_free(dest->priv_data);
 
     memcpy(dest, src, sizeof(*dest));