Prior to this change, when EC_GROUP_get0_generator fails, BN_CTX_end
would get called even though BN_CTX_start hadn't been called yet, in
the case where the caller-supplied |ctx| is not NULL.

Change-Id: I6f728e74f0167193891cdb6f122b20b0770283dc
Reviewed-on: https://boringssl-review.googlesource.com/4271
Reviewed-by: Adam Langley <agl@google.com>