avcodec/avpacket: fix leak on realloc in av_packet_add_side_data()

If realloc fails, the pointer is overwritten and the previously allocated
buffer is leaked, which goes against the expected behavior of keeping the
packet unchanged in case of error.

Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

(cherry picked from commit 574929d8)

libavcodec/avpacket.c

@@ -295,16 +295,17 @@ FF_ENABLE_DEPRECATION_WARNINGS
 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;
 
     if ((unsigned)elems + 1 > INT_MAX / sizeof(*pkt->side_data))
         return AVERROR(ERANGE);
 
-    pkt->side_data = av_realloc(pkt->side_data,
-                                (elems + 1) * sizeof(*pkt->side_data));
-    if (!pkt->side_data)
+    tmp = av_realloc(pkt->side_data, (elems + 1) * sizeof(*tmp));
+    if (!tmp)
         return AVERROR(ENOMEM);
 
+    pkt->side_data = tmp;
     pkt->side_data[elems].data = data;
     pkt->side_data[elems].size = size;
     pkt->side_data[elems].type = type;