EC_GROUP_cmp should return zero if the groups match.

(I got this wrong when reading the OpenSSL code.)

(Cherry-picked form upstream BoringSSL's
7c21925a10d451ed13ab201e0161dea40b974397)

Change-Id: Icedaa45c9d17e7c2b95fa5be1f7c0bf41cac0880
3 files changed