Reference: Table 8: Interpretation of valid BITPIX value from FITS standard 4.0
Fixes: runtime error: division by zero
Fixes: 14581/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FITS_fuzzer-5652382425284608

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