fix possible memory leaks in CWallet::EncryptWallet

- add missing deletes for pwalletdbEncryption
- add an assert before trying to reserve memory for pwalletdbEncryption
- add a destructor to CWallet, which ensures deletion of
  pwalletdbEncryption on object destruction

src/wallet.h

@@ -143,6 +143,7 @@ public:
     {
         SetNull();
     }
+
     CWallet(std::string strWalletFileIn)
     {
         SetNull();
@@ -150,6 +151,13 @@ public:
         strWalletFile = strWalletFileIn;
         fFileBacked = true;
     }
+
+    ~CWallet()
+    {
+        delete pwalletdbEncryption;
+        pwalletdbEncryption = NULL;
+    }
+
     void SetNull()
     {
         nWalletVersion = FEATURE_BASE;