avcodec/qdm2: Check checksum_size for 0

Fixes: Infinite loop
Fixes: 15337/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_QDM2_fuzzer-5757428949319680

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 7b2ebf89a4)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/qdm2.c

@@ -1727,8 +1727,8 @@ static av_cold int qdm2_decode_init(AVCodecContext *avctx)
     extradata += 4;
 
     s->checksum_size = AV_RB32(extradata);
-    if (s->checksum_size >= 1U << 28) {
-        av_log(avctx, AV_LOG_ERROR, "data block size too large (%u)\n", s->checksum_size);
+    if (s->checksum_size >= 1U << 28 || !s->checksum_size) {
+        av_log(avctx, AV_LOG_ERROR, "data block size invalid (%u)\n", s->checksum_size);
         return AVERROR_INVALIDDATA;
     }