emulators/virtualbox-ose-additions-legacy: Import improvements from non legacy port

Import changes from commit 1b2394551c0385 to legacy port:

In the virtualbox virtual filesystem code we ship as a patch some
code in an else block is commented out. This produces a code path
in which a pointer variable is dereferenced in an unassigned state,
causing random crashes.

Lacking a better fix, give a default value of NULL to the pointer,
which at least avoids the random pointer dereference issue.

PR: 255386

Alsso import fix for building on recent head from cec55f41e10f13:

Fix build after head commit b4a58fbf640409a1 (vfs: remove cn_thread)

MFH: 2021Q4
(cherry picked from commit c43e12a46f329c2ff336f06c61a6eb6be0ac9108)