Fix memory leak in ssl3_get_cert_verify.

Any ssl3_get_* function that takes ownership of something before the
ssl_get_message call can't early-return without cleanup work.

This fixes valgrind on ClientAuth-Server-Async.

Change-Id: Ie7f0b37cac4d4bb7e06c00bae091fee0386c22da
Reviewed-on: https://boringssl-review.googlesource.com/1413
Reviewed-by: Adam Langley <agl@google.com>