Avoid a multiplication in |ecp_nistz256_get_affine| when |x| is NULL.
This is purely hypothetical, as in real life nobody cares about the
|y| component without also caring about the |x| component, but it
clarifies the code and makes a future change clearer.
Change-Id: Icaa4de83c87b82a8e68cd2942779a06e5db300c3
Reviewed-on: https://boringssl-review.googlesource.com/7588
Reviewed-by: David Benjamin <davidben@google.com>