avcodec/fits: Check gcount and pcount being non negative

Fixes: signed integer overflow: 9223372036854775807 - -30069403896 cannot be represented in type 'long'
Fixes: 30046/clusterfuzz-testcase-minimized-ffmpeg_dem_FITS_fuzzer-5807144773484544

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 c000a91288)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/fits.c

@@ -205,8 +205,12 @@ int avpriv_fits_header_parse_line(void *avcl, FITSHeader *header, const uint8_t
         } else if (!strcmp(keyword, "GROUPS") && sscanf(value, "%c", &c) == 1) {
             header->groups = (c == 'T');
         } else if (!strcmp(keyword, "GCOUNT") && sscanf(value, "%"SCNd64"", &t) == 1) {
+            if (t < 0 || t > INT_MAX)
+                return AVERROR_INVALIDDATA;
             header->gcount = t;
         } else if (!strcmp(keyword, "PCOUNT") && sscanf(value, "%"SCNd64"", &t) == 1) {
+            if (t < 0 || t > INT_MAX)
+                return AVERROR_INVALIDDATA;
             header->pcount = t;
         }
         dict_set_if_not_null(metadata, keyword, value);