Fix crash on kitty when clicking in scrolled command line

When the command line occupies the whole screen such that no prompt
line is shown, we crash due to overflow. Fix that.

Fixes #12178

src/screen.rs

@@ -642,7 +642,7 @@ pub fn offset_in_cmdline_given_cursor(
                 );
                 0
             })
-            .max(self.actual.visible_prompt_lines - 1)
+            .max(self.actual.visible_prompt_lines.saturating_sub(1))
             .min(self.actual.line_count() - 1);
         let line = self.actual.line(y);
         let x = viewport_position.x.max(line.indentation);