commit b1ab211d (MDEV-15053)
introduced the code to unfix the block earlier in buf_block_t::unfix().
After unfixing the block, InnoDB can withdraw the block from the
buffer pool and deallocate it while doing buffer pool resizing.
So subsequent assert could leads to uninitialized memory access of
block.

buf_block_t::unfix(): Unfix the block after checking the assert.