ssl: move Certificate handshake dissection to ssl-utils

There are no dissection differences between DTLS and SSL, so move to
ssl-utils. While at it, skip dissection when the tree is NULL, remove
the plural from certificates length (the length is always larger than
ome), fix type of "ti" for dtls, get rid of "failsafe" subtree
assignment in ssl, get rid of tvb_ensure_bytes_exist in ssl.

Unrelated changes: fix param (tvb vs sub_tvb) for DTLS KeyExchange
handshake messages.

Change-Id: Iecaa45a7a601e55a52aa16180cf219a122fbe95a
Reviewed-on: https://code.wireshark.org/review/2985
Reviewed-by: Anders Broman <a.broman58@gmail.com>