hyperv/stor: Fix the NULL pointer dereference

Reported by:        Netapp
Submitted by:       Hongjiang Zhang <honzhan microsoft com>
Reviewed by:        adrian, sephe, Dexuan Cui <decui microsoft com>
Approved by:        adrian (mentor)
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5097