avcodec/parser: Check next index validity in ff_combine_frame()

Fixes: out of array access
Fixes: 15522/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DNXHD_fuzzer-5747756078989312

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

libavcodec/parser.c

@@ -245,6 +245,9 @@ int ff_combine_frame(ParseContext *pc, int next,
     for (; pc->overread > 0; pc->overread--)
         pc->buffer[pc->index++] = pc->buffer[pc->overread_index++];
 
+    if (next > *buf_size)
+        return AVERROR(EINVAL);
+
     /* flush remaining if EOF */
     if (!*buf_size && next == END_NOT_FOUND)
         next = 0;