Fix for Issue2515: Don't crash when trying to fetch data from a closed cursor.

Modules/_sqlite/cursor.c

@@ -851,6 +851,7 @@ PyObject* cursor_iternext(Cursor *self)
         next_row = next_row_tuple;
     }
 
+    if (self->statement) {
         rc = _sqlite_step_with_busyhandler(self->statement->st, self->connection);
         if (rc != SQLITE_DONE && rc != SQLITE_ROW) {
             Py_DECREF(next_row);
@@ -861,6 +862,7 @@ PyObject* cursor_iternext(Cursor *self)
         if (rc == SQLITE_ROW) {
             self->next_row = _fetch_one_row(self);
         }
+    }
 
     return next_row;
 }