matroskadec: add assert on lack of overflow in pkt_size+offset

currently a overflow there should be impossible but future changes to
the code could easily introduce a bug that no longer limits the 2
values sufficiently so better protect it via av_assert.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -2053,6 +2053,7 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, uint8_t *data,
                     offset = matroska_decode_buffer(&pkt_data,&pkt_size, track);
                     if (offset < 0)
                         continue;
+                    av_assert0(offset + pkt_size >= pkt_size);
                 }
 
                 pkt = av_mallocz(sizeof(AVPacket));