[PATCH v2] ieee1275: Fix crash in of_path_of_nvme when of_path is empty

[John Paul Adrian Glaubitz]

Hi!

We have recently backported the upstream changes to Debian's grub2 package
to support modern SPARC hardware for Debian's sparc64 port.

One of the changes introduced a regression on systems with NVME devices which
are not based on Open Firmware. On these systems, find_obppath returns an
empty string and trying to append the disk name to that empty name results
in a crash in of_path_of_nvme.

Fix that by making sure of_path isn't empty and if it is, just return
the disk name in of_path_of_nvme.

Debian bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=891773

Thanks,
Adrian