compact arena before raising exception; also reserve a few slots to allocate exception objects

src/gc.c

@@ -261,12 +261,24 @@ 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;
+  }
   mrb->arena[mrb->arena_idx++] = p;
   memset(p, 0, sizeof(RVALUE));
-  if (mrb->arena_idx >= MRB_ARENA_SIZE) {
-    /* arena overflow error */
-    mrb_raise(mrb, E_TYPE_ERROR, "arena overflow error");
-  }
   p->tt = ttype;
   p->c = cls;
   paint_partial_white(mrb, p);

src/load.c

@@ -538,7 +538,6 @@ mrb_read_irep(mrb_state *mrb, const char *bin)
   }
 
   mrb->irep_len += nirep;
-
 error_exit:
   mrb_gc_arena_restore(mrb, ai);
   if (ret != MRB_DUMP_OK) {