avcodec/fits: Check bitpix

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>

libavcodec/fits.c

@@ -138,6 +138,17 @@ int avpriv_fits_header_parse_line(void *avcl, FITSHeader *header, const uint8_t
     case STATE_BITPIX:
         CHECK_KEYWORD("BITPIX");
         CHECK_VALUE("BITPIX", bitpix);
+
+        switch(header->bitpix) {
+        case   8:
+        case  16:
+        case  32: case -32:
+        case  64: case -64: break;
+        default:
+            av_log(avcl, AV_LOG_ERROR, "invalid value of BITPIX %d\n", header->bitpix); \
+            return AVERROR_INVALIDDATA;
+        }
+
         dict_set_if_not_null(metadata, keyword, value);
 
         header->state = STATE_NAXIS;