Simplify. Temporary variable "i" is removed.

src/vm.c

@@ -57,10 +57,8 @@ The value below allows about 60000 recursive calls in the simplest case. */
 static inline void
 stack_copy(mrb_value *dst, const mrb_value *src, size_t size)
 {
-  size_t i;
-
-  for (i = 0; i < size; i++) {
-    dst[i] = src[i];
+  while (size-- > 0) {
+    *dst++ = *src++;
   }
 }