This change removes the use of |X509_get_pubkey| from the TLS <= 1.2
code. That function is replaced with a shallow parse of the certificate
to extract the public key instead.

Change-Id: I8938c6c5a01b32038c6b6fa58eb065e5b44ca6d2
Reviewed-on: https://boringssl-review.googlesource.com/12707
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>