Leave hash->ht to be NULL if initial capacity is zero.

src/hash.c

@@ -144,7 +144,10 @@ mrb_hash_new_capa(mrb_state *mrb, mrb_int capa)
 
   h = (struct RHash*)mrb_obj_alloc(mrb, MRB_TT_HASH, mrb->hash_class);
   /* khash needs 1/4 empty space so it is not resized immediately */
-  h->ht = kh_init_size(ht, mrb, (khint_t)(capa*4/3));
+  if (capa == 0)
+    h->ht = 0;
+  else
+    h->ht = kh_init_size(ht, mrb, (khint_t)(capa*4/3));
   h->iv = 0;
   return mrb_obj_value(h);
 }