avformat/mpc8: correct integer overflow in mpc8_parse_seektable()

Fixes: signed integer overflow: -4683718486770919638 * 2 cannot be represented in type 'long'
Fixes: 26704/clusterfuzz-testcase-minimized-ffmpeg_dem_MPC8_fuzzer-6327056939614208
Fixes: 27550/clusterfuzz-testcase-minimized-ffmpeg_dem_MPC8_fuzzer-6259212652642304

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpc8.c

@@ -182,7 +182,7 @@ static void mpc8_parse_seektable(AVFormatContext *s, int64_t off)
         t += get_bits(&gb, 12);
         if(t & 1)
             t = -(t & ~1);
-        pos = (t >> 1) + ppos[0]*2 - ppos[1];
+        pos = (t >> 1) + (uint64_t)ppos[0]*2 - ppos[1];
         av_add_index_entry(s->streams[0], pos, (int64_t)i << seekd, 0, 0, AVINDEX_KEYFRAME);
         ppos[1] = ppos[0];
         ppos[0] = pos;