avformat/nutdec: Fix integer overflow in count computation

Note, the value is checked a few lines later already

Fixes: signed integer overflow: -440402016 - 1879048064 cannot be represented in type 'int'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_NUT_fuzzer-6603876618469376

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 0014249fd9)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/nutdec.c

@@ -260,7 +260,7 @@ static int decode_main_header(NUTContext *nut)
         if (tmp_fields > 5)
             count = ffio_read_varlen(bc);
         else
-            count = tmp_mul - tmp_size;
+            count = tmp_mul - (unsigned)tmp_size;
         if (tmp_fields > 6)
             get_s(bc);
         if (tmp_fields > 7)