bpo-41815: SQLite: segfault if backup called on closed database (GH-22322)

# [bpo-41815](): SQLite: fix segfault if backup called on closed database

Attempting to backup a closed database will trigger segfault:

```python
import sqlite3
target = sqlite3.connect(':memory:')
source = sqlite3.connect(':memory:')
source.close()
source.backup(target)
```