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/base58.h

@@ -17,6 +17,8 @@
 
 #include <string>
 #include <vector>
+#include <openssl/crypto.h> // for OPENSSL_cleanse()
+
 #include "bignum.h"
 #include "key.h"
 #include "script.h"
@@ -190,7 +192,7 @@ protected:
     {
         // zero the memory, as it may contain sensitive data
         if (!vchData.empty())
-            memset(&vchData[0], 0, vchData.size());
+            OPENSSL_cleanse(&vchData[0], vchData.size());
     }
 
     void SetData(int nVersionIn, const void* pdata, size_t nSize)
@@ -221,7 +223,7 @@ public:
         vchData.resize(vchTemp.size() - 1);
         if (!vchData.empty())
             memcpy(&vchData[0], &vchTemp[1], vchData.size());
-        memset(&vchTemp[0], 0, vchTemp.size());
+        OPENSSL_cleanse(&vchTemp[0], vchData.size());
         return true;
     }