is not null then hi_water must exceed lo_water and both must be > 0.
Now requires hi_water >= lo_water, and they can both be 0.  Not allowing
them to be equal was just an unnecessary restriction, and letting them be 0
is useful for the case where the context has had non-zero values set, went
over hi_water and called the callback, and then wanted to remove limits
(perhaps because of a reconfiguration).  This allows the callback to
be signaled as ISC_MEM_LOWATER on the next isc_mem_put(), which the caller
might need to terminate any outstanding action that was triggered by
the hi_water.