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/crypter.cpp

@@ -24,8 +24,8 @@ bool CCrypter::SetKeyFromPassphrase(const SecureString& strKeyData, const std::v
 
     if (i != (int)WALLET_CRYPTO_KEY_SIZE)
     {
-        memset(&chKey, 0, sizeof chKey);
-        memset(&chIV, 0, sizeof chIV);
+        OPENSSL_cleanse(chKey, sizeof(chKey));
+        OPENSSL_cleanse(chIV, sizeof(chIV));
         return false;
     }