mempool_destroy() already checks for a NULL value being passed in, this
eliminates duplicate checks.

This was caught by running make coccicheck M=block/ on linus' tree on
commit 77ede3a0 (current head as of this
patch).
Reviewed-by: Kyle Fortin <kyle.fortin@oracle.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Tim Hansen <devtimhansen@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>