[reSIProcate] Crash bug in the DnsSub.cxx and AresDns.cxx

[Karlsson <boost.regex@xxxxxxxxx>]

In the 1.8.5, I got the crashed often, once I debug it, I think there is a bug:

In the DnsSub.cxx:

DnsStub::~DnsStub()

{

   for (set<Query*>::iterator it = mQueries.begin(); it != mQueries.end(); ++it)

   {

      delete *it;

   }

   setPollGrp(0);

   delete mDnsProvider;

}

The parameter of  "setPollGrp" function is 0, so in the AresDns::setPollGrp(FdPollGrp *grp) function, if the 

mPollItems is not empty, will cause AresDnsPollItem::resetPollGrp crash.