Bug#891773: [PATCH] ieee1275: Fix crash in of_path_of_nvme when of_path is empty

[John Paul Adrian Glaubitz]

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
but not 

    The of_path_of_nvme function (commit 2391d57, ieee1275: add nvme
    support within ofpath) introduced a functional regression:
    
    On systems where of_path is empty, i.e. non-OpenFirmware
    machines, grub-probe crashes in of_path_of_nvme when trying
    to append the disk name to an empty of_path.