nvedit: simplify do_env_indirect()

Instead of calling env_get(from) up to three times, just do it once,
computing the value we will put into 'to' and error out if that is
NULL (i.e. no 'from' variable and no default provided).

No functional change.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>