chat: guard pop_line against empty list

Small defensive improvement - only pop markers and invalidate scroll
if lines.pop() actually removed something.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/chat.rs

@@ -368,10 +368,11 @@ impl PaneState {
     }
 
     fn pop_line(&mut self) {
-        self.lines.pop();
+        if self.lines.pop().is_some() {
             self.markers.pop();
             self.scroll.invalidate_from(self.lines.len());
         }
+    }
 
     fn scroll_up(&mut self, n: u16) {
         self.scroll.scroll_up(n);