[reSIProcate] c-ares Coverity improvements

[Daniel Pocock]

The c-ares community have been doing some Coverity scans and fixing some
potential security and stability issues in the code, it is discussed on
their mailing list:

http://c-ares.haxx.se/mail/c-ares-archive-2014-09/

It may be useful to update or merge into the reSIProcate copy of the
c-ares code to benefit from that work

https://github.com/resiprocate/resiprocate/tree/master/rutil/dns/ares