Clear currently opened database file name when closing database

This avoids possible confusions when getting the currently opened
database file. After closing a database this would return the previously
opened file when in fact no file is opened.

See issue #2334.

src/sqlitedb.cpp

@@ -721,6 +721,7 @@ bool DBBrowserDB::close()
         if(sqlite3_close(_db) != SQLITE_OK)
             qWarning() << tr("Database didn't close correctly, probably still busy");
         _db = nullptr;
+        curDBFilename.clear();
     }
 
     schemata.clear();