avformat/rtpdec_xiph: Check upper bound on len in xiph_handle_packet()

Larger packets are not supported and would cause problems later
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit aa516993)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtpdec_xiph.c

@@ -112,7 +112,7 @@ static int xiph_handle_packet(AVFormatContext *ctx, PayloadContext *data,
         return data->split_pkts > 0;
     }
 
-    if (len < 6) {
+    if (len < 6 || len > INT_MAX/2) {
         av_log(ctx, AV_LOG_ERROR, "Invalid %d byte packet\n", len);
         return AVERROR_INVALIDDATA;
     }