don't use memset() in privacy/security relevant code parts

As memset() can be optimized out by a compiler it should not be used in
privacy/security relevant code parts. OpenSSL provides the safe
OPENSSL_cleanse() function in crypto.h, which perfectly does the job of
clean and overwrite data.

For details see: http://www.viva64.com/en/b/0178/

- change memset() to OPENSSL_cleanse() where appropriate
- change a hard-coded number from netbase.cpp into a sizeof()

src/netbase.cpp

@@ -297,7 +297,7 @@ bool ConnectSocket(const CService &addrDest, SOCKET& hSocketRet, int nTimeout)
 
 void CNetAddr::Init()
 {
-    memset(ip, 0, 16);
+    memset(ip, 0, sizeof(ip));
 }
 
 void CNetAddr::SetIP(const CNetAddr& ipIn)