Reset pblocktree before deleting LevelDB file

Github-Pull: #12401
Rebased-From: a8b5d20f4f171828b2bd70ab2405c42b1e452e5b
Tree-SHA512: 3a87b6113283c3588f46bb5c725ec33ac639e2f91c589b5c0eb4375e3d23bd6c18e7ba96faf70be2afea86d8e6252bf4dbcf9c9ed166ce2d49846ff947a36d2e

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) {