If the default range of [0, ~0] is given, then (~0 - 0) + 1 == 0. This
in turn will cause any allocation of non-zero size to fail. Zero-sized
allocations are prohibited, so add a KASSERT to this effect.

History indicates it is part of the original rman code.  This bug may in
fact be older than some contributors.

Reviewed by:	mhorne
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D30280