Add missing cs_main lock to VerifyDB

Fixes issue #4139.

src/main.cpp

@@ -2875,6 +2875,7 @@ bool static LoadBlockIndexDB()
 
 bool VerifyDB(int nCheckLevel, int nCheckDepth)
 {
+    LOCK(cs_main);
     if (chainActive.Tip() == NULL || chainActive.Tip()->pprev == NULL)
         return true;