avcodec/fits: Check bscale

Fixes: division by 0
Fixes: 26208/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FITS_fuzzer-6270472117026816

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 c2ccd76fd0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/fits.c

@@ -187,6 +187,8 @@ int avpriv_fits_header_parse_line(void *avcl, FITSHeader *header, const uint8_t
             header->blank = t;
             header->blank_found = 1;
         } else if (!strcmp(keyword, "BSCALE") && sscanf(value, "%lf", &d) == 1) {
+            if (d <= 0)
+                return AVERROR_INVALIDDATA;
             header->bscale = d;
         } else if (!strcmp(keyword, "BZERO") && sscanf(value, "%lf", &d) == 1) {
             header->bzero = d;