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.h

@@ -76,8 +76,8 @@ public:
 
     void CleanKey()
     {
-        memset(&chKey, 0, sizeof chKey);
-        memset(&chIV, 0, sizeof chIV);
+        OPENSSL_cleanse(chKey, sizeof(chKey));
+        OPENSSL_cleanse(chIV, sizeof(chIV));
         fKeySet = false;
     }