flow_age_stats: fix invalid conversion

This invalid conversion causes error with gcc14 build

Author: Pavel Siska

flow_age_stats/flow_age_stats.c

@@ -150,7 +150,7 @@ int main(int argc, char **argv)
    TRAP_REGISTER_DEFAULT_SIGNAL_HANDLER();
 
    FILE* out = NULL;
-   int file = NULL;
+   int file = 0;
    /**
     * Handling of arguments
    */