Fix read-past-end-of-buffer in CUPS dissector

Even when the protocol specifies that a string ends in a null-terminator, there
are packets which won't. Therefore:

**the result of tvb_get_ptr SHALL NOT be treated as a null-terminated string**

We need to post that in big neon lettering somewhere obvious.

Bug: 10353
Change-Id: Ie30cd13353a9830311e573e734dd89dc07622c98
Reviewed-on: https://code.wireshark.org/review/3486
Reviewed-by: Evan Huus <eapache@gmail.com>