Fix easy diff check for PoS

COPYING

@@ -1,3 +1,4 @@
+Copyright (c) 2016-2018 The Komodo developers
 Copyright (c) 2016-2017 The Zcash developers
 Copyright (c) 2009-2017 The Bitcoin Core developers
 

src/pow.cpp

@@ -190,6 +190,8 @@ bool CheckProofOfWork(int32_t height,uint8_t *pubkey33,uint256 hash,unsigned int
     }
     if (fNegative || bnTarget == 0 || fOverflow || bnTarget > UintToArith256(params.powLimit))
         return error("CheckProofOfWork(): nBits below minimum work");
+    if ( height >= 4000 && ASSETCHAINS_STAKED != 0 )
+        bnTarget.SetCompact(KOMODO_MINDIFF_NBITS,&fNegative,&fOverflow);
     // Check proof of work matches claimed amount
     if ( UintToArith256(hash) > bnTarget )
     {