patch 8.0.1496: clearing a pointer takes two lines

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)
diff --git a/src/gui_w32.c b/src/gui_w32.c
index 67aa1fc..6596cf8 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -4941,8 +4941,7 @@
 		    char_u		*str = NULL;
 		    static void		*tt_text = NULL;
 
-		    vim_free(tt_text);
-		    tt_text = NULL;
+		    VIM_CLEAR(tt_text);
 
 # ifdef FEAT_GUI_TABLINE
 		    if (gui_mch_showing_tabline()