avcodec/cbs: Allocate more CodedBitstreamUnit at once in cbs_insert_unit()

Fixes: Timeout (85sec -> 0.5sec)
Fixes: 20791/clusterfuzz-testcase-minimized-ffmpeg_BSF_AV1_FRAME_SPLIT_fuzzer-5659537719951360
Fixes: 21214/clusterfuzz-testcase-minimized-ffmpeg_BSF_MPEG2_METADATA_fuzzer-5165560875974656
Fixes: 21247/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_METADATA_fuzzer-5715175257931776

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

libavcodec/cbs.c

@@ -693,11 +693,11 @@ static int cbs_insert_unit(CodedBitstreamContext *ctx,
             memmove(units + position + 1, units + position,
                     (frag->nb_units - position) * sizeof(*units));
     } else {
-        units = av_malloc_array(frag->nb_units + 1, sizeof(*units));
+        units = av_malloc_array(frag->nb_units*2 + 1, sizeof(*units));
         if (!units)
             return AVERROR(ENOMEM);
 
-        ++frag->nb_units_allocated;
+        frag->nb_units_allocated = 2*frag->nb_units_allocated + 1;
 
         if (position > 0)
             memcpy(units, frag->units, position * sizeof(*units));