efi: check *path non-null before grub_strrchr

efi: check *path non-null before grub_strrchr

The EFI version of grub_machine_get_bootlocation crops the boot image
name back to the last / in order to get a directory path. However, it
does not check that *name is actually set before calling grub_strrchr
to do this, and neither does grub_strrchr before dereferencing a NULL
pointer.

Parent function, grub_set_prefix_and_root, does check the pointer
before using.