efi_loader: correct handling of EFI binary return code

We should not try to remove protocol interfaces from a NULL handle.
efi_run_image() should always return the return code of the executed EFI
binary.

Fixes: 6422820a ("efi_loader: split unrelated code from efi_bootmgr.c")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>