patch 8.2.0173: build fails with old compiler

Problem:    Build fails with old compiler.
Solution:   Do not use anonymous unions. (John Marriott)
diff --git a/src/channel.c b/src/channel.c
index 3d891e8..3fdd312 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4166,8 +4166,8 @@
 
 	    // Move the item from the list and then change the type to
 	    // avoid the value being freed.
-	    *rettv = list->lv_last->li_tv;
-	    list->lv_last->li_tv.v_type = VAR_NUMBER;
+	    *rettv = list->lv_u.mat.lv_last->li_tv;
+	    list->lv_u.mat.lv_last->li_tv.v_type = VAR_NUMBER;
 	    free_tv(listtv);
 	}
     }