If we fail to write to a vnode during a ktrace write, then we drop all
other references to that vnode as a trace vnode in other processes as well
as in any pending requests on the todo list. Thus, it is possible for a
ktrace request structure to have a NULL ktr_vp when it is destroyed in
ktr_freerequest(). We shouldn't call vrele() on the vnode in that case.

Reported by: bde