sip: prevent too long hostname when resolving

This patch fixes PJSIP crash (assertion) due to too long
hostname given to resolve method.
We have a check for that but as PJSIP may prefix our given
name by such string as "_sip._udp.", the check is not enough.
This patch adds a security marge of 12 bytes to handle that.

Change-Id: Icb47e7d105ef0c8a9a10b1984f5e95bede623f8c
Tuleap: #969