avcodec/avpacket: Improve allocating packets

av_mallocz + av_init_packet leads to the same result as av_mallocz +
av_packet_unref, but faster.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avpacket.c

@@ -54,7 +54,7 @@ AVPacket *av_packet_alloc(void)
     if (!pkt)
         return pkt;
 
-    av_packet_unref(pkt);
+    av_init_packet(pkt);
 
     return pkt;
 }