don't request terminal completions if cursor index < 0

[meganrogge]

fix #238380

[meganrogge]

After a command is executed, we set it to -1. When there's next input, if there's prompt input detection IE shell integration, we set the value again. Thus, when it's set to -1, it means we do not have knowledge of where the cursor is.

Running ls then typing a char, which is detected bc shell integration still works

Running bash then typing a char, which is not detected bc no shell integration is present, so it remains -1