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/netbase.cpp

@@ -820,10 +820,10 @@ std::vector<unsigned char> CNetAddr::GetGroup() const
     return vchRet;
 }
 
-int64 CNetAddr::GetHash() const
+uint64 CNetAddr::GetHash() const
 {
     uint256 hash = Hash(&ip[0], &ip[16]);
-    int64 nRet;
+    uint64 nRet;
     memcpy(&nRet, &hash, sizeof(nRet));
     return nRet;
 }