Remove unnecessary NULL initializations in ssl_cert_dup.

A casual grep would suggest this function has the same problems as
CVE-2015-0291, but the structure is memset to 0, so the calls are unnecessary.
Also use BUF_memdup rather than an OPENSSL_malloc + mempcy pair.

Change-Id: Id605374d99cff32e2dccb7f9b8a9da226faf7715
Reviewed-on: https://boringssl-review.googlesource.com/4051
Reviewed-by: Adam Langley <agl@google.com>