1. Check for the presence of the private key before allocating or
   computing anything.
2. Check the return value of |BN_CTX_get|.
3. Don't bother computing the Y coordinate since it is not used.
4. Remove conditional logic in cleanup section.

Change-Id: I4d8611603363c7e5d16a8e9f1d6c3a56809f27ae
Reviewed-on: https://boringssl-review.googlesource.com/6171
Reviewed-by: Adam Langley <alangley@gmail.com>