efi_loader: error handling in efi_load_image()

If a failure occurs when trying to load an image, it is insufficient
to free() the EFI object. We must remove it from the object list,
too. Otherwise a use after free will occur the next time we
iterate over the object list.

Furthermore errors in setting up the image should be handled.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>