hyperv: Fix Coverity issues in the Hyper-V driver

Summary:
Coverity reports the use of two uninitialized variables in the hyperv
driver. Examination shows the variables can be accessed while
uninitialized in error cases. Fix both.

CID: 1365235
CID: 1365236
Sponsored by: Dell Technologies
MFC after: 1w

Test Plan: Changes in use at $WORK

Reviewers: robert.herndon_dell.com, vangyzen, bret_ketchum_dell.com

Subscribers: imp, badger

Differential Revision: https://reviews.freebsd.org/D38551