On close of splashscreen interrupt verifyDB

With the splashscreen being able to be closed it is possible to
shutdown during the lengthy verifyDB method. (Takes about a minute
on my machine). This change allows us to shutdown much sooner.

Github-Pull: #5557

src/main.cpp

@@ -3017,6 +3017,8 @@ bool CVerifyDB::VerifyDB(CCoinsView *coinsview, int nCheckLevel, int nCheckDepth
             } else
                 nGoodTransactions += block.vtx.size();
         }
+        if (ShutdownRequested())
+            return true;
     }
     if (pindexFailure)
         return error("VerifyDB() : *** coin database inconsistencies found (last %i blocks, %i good transactions before that)\n", chainActive.Height() - pindexFailure->nHeight + 1, nGoodTransactions);