Cursor::get_text() is not safe

[dckc]

sqlite3_column_text() can return NULL and get_text() doesn't handle it. For example, these tests will result in a SEGV (status=11):

    #[test]
    fn get_text_without_step() {
        let db = checked_open();
        let c = checked_prepare(&db, "select 1 + 1");
        assert_eq!(c.get_text(0).as_slice(), "");
    }

    #[test]
    fn get_text_on_bogus_col() {
        let db = checked_open();
        let c = checked_prepare(&db, "select 1 + 1");
        c.step();
        assert_eq!(c.get_text(1).as_slice(), "");
    }