Fix GC: don't increment megablock count, when re-use the megablock in free list

The problem is caused, when `_JHC_JGC_LIMITED_NUM_MEGABLOCK` is enabled.

The heavy-memory-use program call `s_new_megablock` via `s_alloc`. Because `s_new_megablock` has free list, this does'nt always cause a lack of memory.

But `s_new_megablock` is increment mega block count on each call. So, we reach to limit before a lack of memory.