boot1: also check for NULL device

r325681 fixed a NULL pointer dereference on RPi3 caused by a lack of
functionality in uboot's EFI implementation. That rev checked the boot1
load path for NULL but not the load device. In practice if the former
works the latter will as well, but improve correctness by checking each
separately.

Submitted by: Keith White <kwhite@eecs.uottawa.ca>
Reported by: jhb
MFC after: 5 days
MFC with: r325681
Sponsored by: The FreeBSD Foundation