patch 8.2.1221: memory leak when updating popup window

Problem:    Memory leak when updating popup window.
Solution:   Clear search highlighting.
diff --git a/src/popupwin.c b/src/popupwin.c
index 59a0bdf..296441b 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -3855,6 +3855,11 @@
 	// Back to the normal zindex.
 	screen_zindex = 0;
     }
+
+#if defined(FEAT_SEARCH_EXTRA)
+    // In case win_update() called start_search_hl().
+    end_search_hl();
+#endif
 }
 
 /*