Fixed incorrect return code handling in ssl3_final_finish_mac. Based on an original patch by Joel Sing (OpenBSD) who also originally identified the issue.
The OPENSSL_cleanse() in aes_gcm_cleanup() only cleans the gcm field of the
EVP_AES_GCM_CTX, leaving the AES key untouched - clean the entire context,
rather than just part of it.

Patch by beck@ and miod@ (OpenBSD).