Reject invalid pubkeys when reading ckey items from the wallet.

This makes the behavior more consistent with key objects and will
 reject some corrupted pubkeys (e.g. zero length).

src/wallet/walletdb.cpp

@@ -512,8 +512,13 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
         }
         else if (strType == "ckey")
         {
-            vector<unsigned char> vchPubKey;
+            CPubKey vchPubKey;
             ssKey >> vchPubKey;
+            if (!vchPubKey.IsValid())
+            {
+                strErr = "Error reading wallet database: CPubKey corrupt";
+                return false;
+            }
             vector<unsigned char> vchPrivKey;
             ssValue >> vchPrivKey;
             wss.nCKeys++;