avformat/rpl: The associative law doesnt hold for signed integers in C

Add () to avoid undefined behavior
Fixes: signed integer overflow: 9223372036854775790 + 57 cannot be represented in type 'long'
Fixes: 34983/clusterfuzz-testcase-minimized-ffmpeg_dem_RPL_fuzzer-5765822923538432

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

libavformat/rpl.c

@@ -103,7 +103,7 @@ static AVRational read_fps(const char* line, int* error)
         // Truncate any numerator too large to fit into an int64_t
         if (num > (INT64_MAX - 9) / 10 || den > INT64_MAX / 10)
             break;
-        num  = 10 * num + *line - '0';
+        num  = 10 * num + (*line - '0');
         den *= 10;
     }
     if (!num)