contrib: oversize PJ_ICE_MAX_CAND to prevent crash

PJ_ICE_MAX_CAND was 32 (already increased from 16, value from upstream)
and it's not enough to handle cases with numerous local/remote candidates
like the case when multiple hosts in IPv6 and IPv4 exists.

The value is increased to 256, I hope it's enough...
(my tests show I need 40 entries with 5 local host IPs)

Change-Id: I28aba472a5c19497cbe64b764084841ff1b9e212
Tuleap: #1601