efi_loader: Call Exit() on return from payload in StartImage()

When a UEFI payload just returns instead of calling the Exit() callback,
we handle that in efi_do_enter() and call Exit on its behalf, so that
the loaded_image->exit_status value is correct.

We were missing that logic in StartImage(). Call it there too.

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