Fix a bootlock condition if/when loader_logo is set in loader.conf(5)

Fix a bootlock condition if/when loader_logo is set in loader.conf(5)
NB: This deeply effected HardenedBSD which had a default value set.

Embarassingly, I allowed the `type' primitive to be passed -1/-1 for
c-addr/u stack input (the effect of which is to pull INT_MAX bytes
from character address negative one in which hilarity ensues over a
black screen in full-on bootlock). Much thanks to Shawn Webb [lattera]
for helping me diagnose.

NB: The mode-ending revisions were initially suspected (and reverted)
but proved to be a red-herring. Proper mode endings will be returning.

Thanks to:	lattera (@HardenedBSD [<dot><com>])
Reported by:	lattera
MFC after:	3 days
X-MFC-to:	stable/10