avcodec/fitsdec: Fix division by 0 in size check

Fixes: division by zero
Fixes: 15210/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FITS_fuzzer-5746033243455488

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

libavcodec/fitsdec.c

@@ -143,7 +143,7 @@ static int fits_read_header(AVCodecContext *avctx, const uint8_t **ptr, FITSHead
 
     size = abs(header->bitpix) >> 3;
     for (i = 0; i < header->naxis; i++) {
-        if (header->naxisn[i] > SIZE_MAX / size) {
+        if (size && header->naxisn[i] > SIZE_MAX / size) {
             av_log(avctx, AV_LOG_ERROR, "unsupported size of FITS image");
             return AVERROR_INVALIDDATA;
         }