[sqlite-] prevent creation of ./- file when reading from stdin

[midichef]

If visidata tries to read a SQLite file from standard input, it will load an empty sheet and create the file - in the current directory. This can be seen with cat sample_data/employees.sqlite |vd -f sqlite3

That's because the path for stdin gets resolved as the file - in the current directory.
As far as I can tell, there's no way to read an SQLite file from standard input, at least not with the Python sqlite library. When I try it, using sqlite.connect() with '/dev/stdin', I get this error

Traceback (most recent call last):
  File "/home/midichef/.local/lib/python3.10/site-packages/visidata/threads.py", line 200, in _toplevelTryFunc
    t.status = func(*args, **kwargs)
  File "/home/midichef/.local/lib/python3.10/site-packages/visidata/sheets.py", line 235, in reload
    for r in self.iterload():
  File "/home/midichef/.local/lib/python3.10/site-packages/visidata/loaders/sqlite.py", line 179, in iterload
    for row in SqliteSheet.iterload(self):
  File "/home/midichef/.local/lib/python3.10/site-packages/visidata/loaders/sqlite.py", line 84, in iterload
    with self.conn() as conn:
  File "/home/midichef/.local/lib/python3.10/site-packages/visidata/loaders/sqlite.py", line 58, in conn
    con = sqlite3.connect(url, uri=True, **self.options.getall('sqlite_connect_'))
sqlite3.OperationalError: disk I/O error

So this commit just disables reading any sqlite file from standard input.

[saulpw]

Heh, okay. How did you find this one? I never would have thought to pipe a sqlite db into vd.

[midichef]

Yeah, it's an odd thing to try! I was trying to find a hack to effectively make the sqlite db read-only, in response to #1805