avcodec/h264dec: Disable forced small_padding on flag2 fast

Fixes: 20978/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5746381832847360

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

libavcodec/h264dec.c

@@ -603,7 +603,7 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size)
     }
 
     ret = ff_h2645_packet_split(&h->pkt, buf, buf_size, avctx, h->is_avc, h->nal_length_size,
-                                avctx->codec_id, avctx->flags2 & AV_CODEC_FLAG2_FAST, 0);
+                                avctx->codec_id, 0, 0);
     if (ret < 0) {
         av_log(avctx, AV_LOG_ERROR,
                "Error splitting the input into NAL units.\n");