MFC r343473:
geom_uzip(4): move NULL pointer KASSERT check before it is dereferenced

PR: 203499
Submitted by: <chadf@triularity.org>

MFC r343475:
geom_uzip(4): set 'gp != NULL' assertion on top of the function

There was yet another access to this variable in g_trace() few
lines upper.

PR: 203499
Reported by: cem