Make CNetAddr::GetHash() return an unsigned val.

This prevents an undefined operation in main.cpp, when shifting the hash value
left by 32 bits.
Shifting a signed int left into the sign bit is undefined in C++11.

src/main.cpp

@@ -2440,7 +2440,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
                     static uint256 hashSalt;
                     if (hashSalt == 0)
                         RAND_bytes((unsigned char*)&hashSalt, sizeof(hashSalt));
-                    int64 hashAddr = addr.GetHash();
+                    uint64 hashAddr = addr.GetHash();
                     uint256 hashRand = hashSalt ^ (hashAddr<<32) ^ ((GetTime()+hashAddr)/(24*60*60));
                     hashRand = Hash(BEGIN(hashRand), END(hashRand));
                     multimap<uint256, CNode*> mapMix;