bli_pool: add sanity check in bli_pool_free_block()

[hominhquan]

Details:

• In case of pool reinit due to a thread asking for a bigger block, the pool is re-initialized and all blocks are free-ed, including potential ones already checked out and being used elsewhere. Those empty slots are marked with NULL pointer and hence we should skip calling free() on them (with subtraction by offset_size).
• The orphaned blocks and their pointers will be free-ed later when checking in back by the existing check-in mechanism.