bhyve nvme: Fix uninitialized pointer

The Dataset Management code could free an uninitialized pointer if the
device doesn't support the Dataset Management command.

PR: 264548
Reported by: Robert Morris <rtm@lcs.mit.edu>

(cherry picked from commit 7376c08cc67e65ee660d51873129e34745ef5dd4)
(cherry picked from commit c84ec3076e7de10d7475fd9694ad64603e264d88)