Add av_shrink_packet function for use in av_get_packet that reduces pkt->size

and ensures the following padding is correctly initialized to 0.

Originally committed as revision 18378 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2654,6 +2654,14 @@ void av_init_packet(AVPacket *pkt);
  */
 int av_new_packet(AVPacket *pkt, int size);
 
+/**
+ * Reduce packet size, correctly zeroing padding
+ *
+ * @param pkt packet
+ * @param size new size
+ */
+void av_shrink_packet(AVPacket *pkt, int size);
+
 /**
  * @warning This is a hack - the packet memory allocation stuff is broken. The
  * packet is allocated if it was not really allocated.