avcodec/qdm2: The smallest header seems to have 2 bytes so treat 1 as invalid

Fixes: Timeout (217sec -> 2ms)
Fixes: 18488/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_QDM2_fuzzer-5708293662310400

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

libavcodec/qdm2.c

@@ -1704,7 +1704,7 @@ static av_cold int qdm2_decode_init(AVCodecContext *avctx)
     s->group_size = bytestream2_get_be32(&gb);
     s->fft_size = bytestream2_get_be32(&gb);
     s->checksum_size = bytestream2_get_be32(&gb);
-    if (s->checksum_size >= 1U << 28 || !s->checksum_size) {
+    if (s->checksum_size >= 1U << 28 || s->checksum_size <= 1) {
         av_log(avctx, AV_LOG_ERROR, "data block size invalid (%u)\n", s->checksum_size);
         return AVERROR_INVALIDDATA;
     }