mruby-random: fixed wrong fixnum conversion

mrbgems/mruby-random/src/random.c

@@ -79,10 +79,10 @@ get_opt(mrb_state* mrb)
   mrb_get_args(mrb, "|o", &arg);
 
   if (!mrb_nil_p(arg)) {
-    if (!mrb_fixnum_p(arg)) {
+    arg = mrb_check_convert_type(mrb, arg, MRB_TT_FIXNUM, "Fixnum", "to_int");
+    if (mrb_nil_p(arg)) {
       mrb_raise(mrb, E_ARGUMENT_ERROR, "invalid argument type");
     }
-    arg = mrb_check_convert_type(mrb, arg, MRB_TT_FIXNUM, "Fixnum", "to_int");
     if (mrb_fixnum(arg) < 0) {
       arg = mrb_fixnum_value(0 - mrb_fixnum(arg));
     }