avcodec/avpacket: allow only one element per type in packet side data

It was never meant to do otherwise, as av_packet_get_side_data() returns the first
entry it finds of a given type.

Based on code from libavformat's av_stream_add_side_data().
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/avpacket.c

@@ -296,7 +296,18 @@ int av_packet_add_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
                             uint8_t *data, size_t size)
 {
     AVPacketSideData *tmp;
-    int elems = pkt->side_data_elems;
+    int i, elems = pkt->side_data_elems;
+
+    for (i = 0; i < elems; i++) {
+        AVPacketSideData *sd = &pkt->side_data[i];
+
+        if (sd->type == type) {
+            av_free(sd->data);
+            sd->data = data;
+            sd->size = size;
+            return 0;
+        }
+    }
 
     if ((unsigned)elems + 1 > AV_PKT_DATA_NB)
         return AVERROR(ERANGE);