[Wallet] ensure CKeyMetadata.hdMasterKeyID will be cleared during SetNull()

Zcash: modified for zip32

src/wallet/walletdb.h

@@ -91,9 +91,8 @@ public:
     }
     CKeyMetadata(int64_t nCreateTime_)
     {
-        nVersion = CKeyMetadata::CURRENT_VERSION;
+        SetNull();
         nCreateTime = nCreateTime_;
-        hdKeypath.clear();
     }
 
     ADD_SERIALIZE_METHODS;
@@ -114,6 +113,7 @@ public:
         nVersion = CKeyMetadata::CURRENT_VERSION;
         nCreateTime = 0;
         hdKeypath.clear();
+        seedFp.SetNull();
     }
 };