lsm: don't crash with "undefined" profile in images

If the profile is "undefined" render_lsm_profile doesn't render anything,
since there is no need to set a profile. We shouldn't crash in this case.

We never hit this bug because we are careful not to put an "undefined"
profile into the images. But, if someone else edits the images, we
shouldn't crash on restore because of that.

Closes #110

Reported-by: Coverity
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>