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/util.cpp

@@ -156,7 +156,7 @@ void RandAddSeedPerfmon()
     if (ret == ERROR_SUCCESS)
     {
         RAND_add(pdata, nSize, nSize/100.0);
-        memset(pdata, 0, nSize);
+        OPENSSL_cleanse(pdata, nSize);
         printf("RandAddSeed() %lu bytes\n", nSize);
     }
 #endif