gvinum: correct assertions

Pointer addresses are always >= 0. Assert that the value is >= 0
instead.

PR: 207855, 207856
Reviewed by: imp
Reported by: David Binderman
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37677

(cherry picked from commit 87bb53cb538059a3085db1fa4295dde5fcba55fe)