vm_phys: add essential segment bounds check

A lower-bound segment check is necessary in vm_phys_alloc_seg_contig.
Add one.

Reported by: jenkins
Reviewed by: alc
Fixes: da92ecbc0d8f vm_phys: fix seg->end test in alloc_seg_contig
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33945