Reset pblocktree before deleting LevelDB file

src/init.cpp

@@ -1425,6 +1425,9 @@ bool AppInitMain()
                 pcoinsTip.reset();
                 pcoinsdbview.reset();
                 pcoinscatcher.reset();
+                // new CBlockTreeDB tries to delete the existing file, which
+                // fails if it's still open from the previous loop. Close it first:
+                pblocktree.reset();
                 pblocktree.reset(new CBlockTreeDB(nBlockTreeDBCache, false, fReset));
 
                 if (fReset) {