loader: efi devpath api usage should be more aware of NULL pointers

As the efi_devpath_last_node() and efi_devpath_trim() can return NULL
pointers, the consumers of this API should check the the NULL pointers.

Same for efinet_dev_init() using calloc().

Reported by: Robert Mustacchi <rm@joyent.com>
Reviewed by: jhb, allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D9203