label: Fix Label highlight may crash of not a char boundary. (#1574)

crates/story/src/label_story.rs

@@ -113,7 +113,9 @@ impl Render for LabelStory {
                     v_flex()
                         .gap_y_4()
                         .child(Label::new("This is a label").highlights(ht.clone()))
-                        .child(Label::new("这是一个标签").highlights(ht.clone())),
+                        // This case for test match CJK with ASCII, it was has a crash bug before.
+                        // Try to input "AA" to see the highlights effect.
+                        .child(Label::new("AAA中文BB").highlights(ht.clone())),
                 ),
             )
             .child(

crates/ui/src/label.rs

@@ -181,7 +181,7 @@ impl Label {
             ));
         }
 
-        Some(highlights)
+        Some(gpui::combine_highlights(vec![], highlights).collect())
     }
 }