refactor fix_mod

fix unreachable condition
use b

src/numeric.c

@@ -774,17 +774,17 @@ static mrb_value
 fix_mod(mrb_state *mrb, mrb_value x)
 {
   mrb_value y;
-  mrb_int a, b;
+  mrb_int a;
 
   mrb_get_args(mrb, "o", &y);
   a = mrb_fixnum(x);
-  if (mrb_fixnum_p(y) && (b=mrb_fixnum(y)) != 0) {
-    mrb_int mod;
+  if (mrb_fixnum_p(y)) {
+    mrb_int b, mod;
 
-    if (mrb_fixnum(y) == 0) {
+    if ((b=mrb_fixnum(y)) == 0) {
       return mrb_float_value(mrb, NAN);
     }
-    fixdivmod(mrb, a, mrb_fixnum(y), 0, &mod);
+    fixdivmod(mrb, a, b, 0, &mod);
     return mrb_fixnum_value(mod);
   }
   else {