avcodec/h2645_parse: Reduce the initial skipped_bytes_pos size with small inputs

Fixes: OOM
Fixes: 23817/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_METADATA_fuzzer-6300869057576960
Fixes: 28055/clusterfuzz-testcase-minimized-ffmpeg_BSF_TRACE_HEADERS_fuzzer-5706035297517568

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

libavcodec/h2645_parse.c

@@ -467,7 +467,7 @@ int ff_h2645_packet_split(H2645Packet *pkt, const uint8_t *buf, int length,
             memset(pkt->nals + pkt->nals_allocated, 0, sizeof(*pkt->nals));
 
             nal = &pkt->nals[pkt->nb_nals];
-            nal->skipped_bytes_pos_size = 1024; // initial buffer size
+            nal->skipped_bytes_pos_size = FFMIN(1024, extract_length/3+1); // initial buffer size
             nal->skipped_bytes_pos = av_malloc_array(nal->skipped_bytes_pos_size, sizeof(*nal->skipped_bytes_pos));
             if (!nal->skipped_bytes_pos)
                 return AVERROR(ENOMEM);