avcodec/avpacket: Fix off by 5 error

Fixes out of array read
Fixes: mozilla bug 1266129
Found-by: Tyson Smith
Tested-by: Tyson Smith
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avpacket.c

@@ -403,10 +403,12 @@ int av_packet_split_side_data(AVPacket *pkt){
         p = pkt->data + pkt->size - 8 - 5;
         for (i=1; ; i++){
             size = AV_RB32(p);
-            if (size>INT_MAX || p - pkt->data < size)
+            if (size>INT_MAX - 5 || p - pkt->data < size)
                 return 0;
             if (p[4]&128)
                 break;
+            if (p - pkt->data < size + 5)
+                return 0;
             p-= size+5;
         }
 
@@ -417,7 +419,7 @@ int av_packet_split_side_data(AVPacket *pkt){
         p= pkt->data + pkt->size - 8 - 5;
         for (i=0; ; i++){
             size= AV_RB32(p);
-            av_assert0(size<=INT_MAX && p - pkt->data >= size);
+            av_assert0(size<=INT_MAX - 5 && p - pkt->data >= size);
             pkt->side_data[i].data = av_mallocz(size + AV_INPUT_BUFFER_PADDING_SIZE);
             pkt->side_data[i].size = size;
             pkt->side_data[i].type = p[4]&127;