Don't report |ERR_R_MALLOC_FAILURE| on failure of |EC_KEY_new_by_curve_name|.

Change |EC_KEY_new_by_curve_name| to report |ERR_R_MALLOC_FAILURE|
itself, so that reporting of |EC_R_UNKNOWN_GROUP| is not confused by
the caller's addition of a spurious |ERR_R_MALLOC_FAILURE|.

Change-Id: Id3f5364f01eb8e3597bcddd6484bc03d5578befb
Reviewed-on: https://boringssl-review.googlesource.com/4690
Reviewed-by: Adam Langley <agl@google.com>