patch 8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
diff --git a/src/popupmenu.c b/src/popupmenu.c
index c6214d2..b267ec5 100644
--- a/src/popupmenu.c
+++ b/src/popupmenu.c
@@ -1382,7 +1382,7 @@
     FOR_ALL_CHILD_MENUS(menu, mp)
 	if ((mp->modes & mp->enabled & mode) && idx++ == pum_selected)
 	{
-	    vim_memset(&ea, 0, sizeof(ea));
+	    CLEAR_FIELD(ea);
 	    execute_menu(&ea, mp, -1);
 	    break;
 	}