Fix bugs found by Coverity in efipart.c. The Issue is that efi_devpath_last_node() can return NULL pointer, and therefore we should check for it. In real life we really do not expect to see it to happen, so we will just error out from the test. CID: 1371004 Reported by: Coverity Reviewed by: allanjude Approved by: allanjude (mentor) Differential Revision: https://reviews.freebsd.org/D9490