gc.c: combine variable declaration with initialization

Co-authored-by: Claude <noreply@anthropic.com>

src/gc.c

@@ -245,8 +245,7 @@ mrb_calloc(mrb_state *mrb, size_t nelem, size_t len)
 
   if (nelem > 0 && len > 0 &&
       nelem <= SIZE_MAX / len) {
-    size_t size;
-    size = nelem * len;
+    size_t size = nelem * len;
     p = mrb_malloc(mrb, size);
 
     memset(p, 0, size);