Ensure that when a new cursor is opened by OP_OpenDup, any existing cursor

with the same id opened by a previous OP_OpenDup is closed first.

FossilOrigin-Name: 5c188361a91407805c0feb4bf6d3214522ce3e55013efcf63a4613ecd416bcbc

src/btree.c

@@ -4416,6 +4416,7 @@ int sqlite3BtreeCloseCursor(BtCursor *pCur){
     sqlite3_free(pCur->aOverflow);
     sqlite3_free(pCur->pKey);
     sqlite3BtreeLeave(pBtree);
+    pCur->pBtree = 0;
   }
   return SQLITE_OK;
 }