use saturating substraction to calculate Line ranges

src/controller.rs

@@ -177,7 +177,8 @@ impl<'b> Controller<'b> {
                     if let Some(line_changes) = line_changes {
                         for &line in line_changes.keys() {
                             let line = line as usize;
-                            line_ranges.push(LineRange::new(line - context, line + context));
+                            line_ranges
+                                .push(LineRange::new(line.saturating_sub(context), line + context));
                         }
                     }