force room in arena before raising arena overflow error

src/gc.c

@@ -261,21 +261,10 @@ mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
   }
 
   mrb->live++;
-  if (mrb->arena_idx > MRB_ARENA_SIZE - 4) {
-    struct RBasic **p, **q, **e;
-
-    p = q = mrb->arena;
-    e = p + mrb->arena_idx;
-    while (p < e) {
-      if (is_white(*p))
-	*q++ = *p;
-      p++;
-    }
-    if (p == q) {
-      /* arena overflow error */
-      mrb_raise(mrb, E_TYPE_ERROR, "arena overflow error");
-    }
-    mrb->arena_idx = q - mrb->arena;
+  if (mrb->arena_idx > MRB_ARENA_SIZE) {
+    /* arena overflow error */
+    mrb->arena_idx = MRB_ARENA_SIZE - 2; /* force room in arena */
+    mrb_raise(mrb, mrb->eRuntimeError_class, "arena overflow error");
   }
   mrb->arena[mrb->arena_idx++] = p;
   memset(p, 0, sizeof(RVALUE));