avformat/mpc8: Check remaining space in mpc8_parse_seektable()

Fixes: Fixes infinite loop
Fixes: 26704/clusterfuzz-testcase-minimized-ffmpeg_dem_MPC8_fuzzer-6327056939614208

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4f66dd13d0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpc8.c

@@ -178,6 +178,10 @@ static void mpc8_parse_seektable(AVFormatContext *s, int64_t off)
         av_add_index_entry(s->streams[0], pos, i, 0, 0, AVINDEX_KEYFRAME);
     }
     for(; i < size; i++){
+        if (get_bits_left(&gb) < 13) {
+            av_free(buf);
+            return;
+        }
         t = get_unary(&gb, 1, 33) << 12;
         t += get_bits(&gb, 12);
         if(t & 1)