efi: Avoid NULL dereference if FilePath is NULL

efi: Avoid NULL dereference if FilePath is NULL

The UEFI specification allows LoadImage() to be called with a memory
location only and without a device path. In this case FilePath will not be
set in the EFI_LOADED_IMAGE_PROTOCOL.

So in function grub_efi_get_filename() the device path argument may be
NULL. As we cannot determine the device path in this case just return NULL
from the function.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>