Avoid calculating the affine Y coordinate when the caller didn't ask
for it, as occurs, for example, in ECDH.
For symmetry and clarity, avoid calculating the affine X coordinate in
the hypothetical case where the caller only asked for the Y coordinate.
Change-Id: I69f5993fa0dfac8b010c38e695b136cefc277fed
Reviewed-on: https://boringssl-review.googlesource.com/7590
Reviewed-by: David Benjamin <davidben@google.com>