avformat/mpsubdec: Use av_sat_add/sub64() in fracval handling

Fixes: signed integer overflow: 9223372036850000000 + 9000000 cannot be represented in type 'long long'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_MPSUB_fuzzer-665448017480908

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

libavformat/mpsubdec.c

@@ -72,8 +72,8 @@ static int parse_line(const char *line, int64_t *value, int64_t *value2)
                 fracval *= 10;
             for (;p2 - p1 > 7 + 1; p1++)
                 fracval /= 10;
-            if (intval > 0) intval += fracval;
-            else            intval -= fracval;
+            if (intval > 0) intval = av_sat_add64(intval, fracval);
+            else            intval = av_sat_sub64(intval, fracval);
             line += p2;
         } else
             line += p1;