Optimize range check.

src/array.c

@@ -566,10 +566,12 @@ mrb_ary_set(mrb_state *mrb, mrb_value ary, mrb_int n, mrb_value val) /* rb_ary_s
 
   ary_modify(mrb, a);
   /* range check */
-  if (n < 0) n += a->len;
+  if (n < 0) {
+    n += a->len;
     if (n < 0) {
       mrb_raise(mrb, E_INDEX_ERROR, "index %ld out of array", n - a->len);
     }
+  }
   if (a->len <= (int)n) {
     if (a->aux.capa <= (int)n)
       ary_expand_capa(mrb, a, n + 1);
@@ -592,10 +594,12 @@ mrb_ary_splice(mrb_state *mrb, mrb_value ary, mrb_int head, mrb_int len, mrb_val
 
   ary_modify(mrb, a);
   /* range check */
-  if (head < 0) head += a->len;
+  if (head < 0) {
+    head += a->len;
     if (head < 0) {
       mrb_raise(mrb, E_INDEX_ERROR, "index is out of array");
     }
+  }
   tail = head + len;
 
   /* size check */