Use mrb_malloc() instead of mrb_realloc().

src/gc.c

@@ -227,7 +227,7 @@ mrb_calloc(mrb_state *mrb, size_t nelem, size_t len)
       nelem <= SIZE_MAX / len) {
     size_t size;
     size = nelem * len;
-    p = mrb_realloc(mrb, 0, size);
+    p = mrb_malloc(mrb, size);
 
     memset(p, 0, size);
   }