ff_alloc_packet: modify the size of the packet to match the requested size

This will simplify encoders which use this function to request the exact
packet size rather than the maximum size.

libavcodec/internal.h

@@ -120,6 +120,7 @@ int avpriv_unlock_avformat(void);
  *                If avpkt->data is already set, avpkt->size is checked
  *                to ensure it is large enough.
  *                If avpkt->data is NULL, a new buffer is allocated.
+ *                avpkt->size is set to the specified size.
  *                All other AVPacket fields will be reset with av_init_packet().
  * @param size    the minimum required packet size
  * @return        0 on success, negative error code on failure

libavcodec/utils.c

@@ -839,16 +839,14 @@ int ff_alloc_packet(AVPacket *avpkt, int size)
 
     if (avpkt->data) {
         uint8_t *pkt_data;
-        int pkt_size;
 
         if (avpkt->size < size)
             return AVERROR(EINVAL);
 
         pkt_data = avpkt->data;
-        pkt_size = avpkt->size;
         av_init_packet(avpkt);
         avpkt->data = pkt_data;
-        avpkt->size = pkt_size;
+        avpkt->size = size;
         return 0;
     } else {
         return av_new_packet(avpkt, size);