Check result from xio_rdma_mempool_alloc().  If it failed,
what it returns is guaranteed to be not valid.  assert()
is better than a confusing failure later.

Signed-off-by: Marcus Watts <mdw@cohortfs.com>
This is a temporary change to put a bunch more buffer sizes
in the range 3k-131k in the "xio_msgr_noreg_mpool" pool.
With this change, ceph-osd at least doesn't fail quickly.
(Need to figure out what sizes it *does* need, or if it needs
it, and replace this code with something better.)

Signed-off-by: Marcus Watts <mdw@cohortfs.com>