ddb show lockedvnods: avoid trap for ufs vnode under construction

ddb show lockedvnods might find a ufs vnode after it is inserted into
the mount queue in ffs_vgetf but before the dinode is allocated. Avoid
trapping by testing for the dinode pointer.

Reviewed by: markj
Discussed with: mjg
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D52795