Handle failures in ssl3_finish_mac.

It may fail because the BIO_write to the memory BIO can allocate.
Unfortunately, this bubbles up pretty far up now that we've moved the handshake
hash to ssl3_set_handshake_header.

Change-Id: I58884347a4456bb974ac4783078131522167e29d
Reviewed-on: https://boringssl-review.googlesource.com/3483
Reviewed-by: Adam Langley <agl@google.com>