avcodec/avpacket: clear priv in av_init_packet()

This should fix leaving uninitialized pointers in priv which can confuse
user applications.
See: https://github.com/golang/go/issues/14426

Only for release branches
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avpacket.c

@@ -58,6 +58,7 @@ void av_init_packet(AVPacket *pkt)
 #if FF_API_DESTRUCT_PACKET
 FF_DISABLE_DEPRECATION_WARNINGS
     pkt->destruct             = NULL;
+    pkt->priv                 = NULL;
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif
     pkt->buf                  = NULL;