avcodec/avpacket: use av_malloc() to allocate an AVPacket

av_mallocz() is superfluous as get_packet_defaults() is called immediately
after it's allocated, which will initialize the entire struct to default
values.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/avpacket.c

@@ -58,7 +58,7 @@ static void get_packet_defaults(AVPacket *pkt)
 
 AVPacket *av_packet_alloc(void)
 {
-    AVPacket *pkt = av_mallocz(sizeof(AVPacket));
+    AVPacket *pkt = av_malloc(sizeof(AVPacket));
     if (!pkt)
         return pkt;