CWallet: fix nTimeFirstKey init, by making constructor init common code

Don't repeat yourself etc.

src/wallet.h

@@ -143,27 +143,26 @@ public:
 
     CWallet()
     {
-        nWalletVersion = FEATURE_BASE;
-        nWalletMaxVersion = FEATURE_BASE;
-        fFileBacked = false;
-        nMasterKeyMaxID = 0;
-        pwalletdbEncryption = NULL;
-        nOrderPosNext = 0;
-        nNextResend = 0;
-        nLastResend = 0;
-        nTimeFirstKey = 0;
+        SetNull();
     }
     CWallet(std::string strWalletFileIn)
     {
-        nWalletVersion = FEATURE_BASE;
-        nWalletMaxVersion = FEATURE_BASE;
+        SetNull();
+
         strWalletFile = strWalletFileIn;
         fFileBacked = true;
+    }
+    void SetNull()
+    {
+        nWalletVersion = FEATURE_BASE;
+        nWalletMaxVersion = FEATURE_BASE;
+        fFileBacked = false;
         nMasterKeyMaxID = 0;
         pwalletdbEncryption = NULL;
         nOrderPosNext = 0;
         nNextResend = 0;
         nLastResend = 0;
+        nTimeFirstKey = 0;
     }
 
     std::map<uint256, CWalletTx> mapWallet;