lavd: Drop unneeded av_init_packet()s

The input packet is already unref'd by the calling function.

libavdevice/v4l2.c

@@ -861,7 +861,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
 #endif
     int res;
 
-    av_init_packet(pkt);
     if ((res = mmap_read_frame(s1, pkt)) < 0) {
         return res;
     }

libavdevice/x11grab.c

@@ -511,7 +511,6 @@ static int x11grab_read_packet(AVFormatContext *s1, AVPacket *pkt)
         nanosleep(&ts, NULL);
     }
 
-    av_init_packet(pkt);
     pkt->data = image->data;
     pkt->size = s->frame_size;
     pkt->pts  = curtime;