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