Fix a very ancient bug from mid 2015 (dashpay#2021)

Introduced in dashpay@c701839#diff-2e2ff25b7bc057a741bf93c35ae3b624R42 (committed on 10 Jul 2015). Survived all the refactoring for almost 3 years and was revealed by dashpay@525c049#diff-49ff8fea774f647034a130c40d4f3c65R519 (as a part of dashpay#1856) by causing silent crashes on multiple nodes (testnet). v0.12.2.x and earlier branches/versions aren't affected, they simply print meaningless log entry.

Author: UdjinM6

src/governance-object.cpp

@@ -540,7 +540,7 @@ bool CGovernanceObject::IsCollateralValid(std::string& strError, bool& fMissingC
     // RETRIEVE TRANSACTION IN QUESTION
 
     if(!GetTransaction(nCollateralHash, txCollateral, Params().GetConsensus(), nBlockHash, true)){
-        strError = strprintf("Can't find collateral tx %s", txCollateral->ToString());
+        strError = strprintf("Can't find collateral tx %s", nCollateralHash.ToString());
         LogPrintf("CGovernanceObject::IsCollateralValid -- %s\n", strError);
         return false;
     }