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_photon.c b/src/gui_photon.c
index fcfb4f6..327b356 100644
--- a/src/gui_photon.c
+++ b/src/gui_photon.c
@@ -1040,8 +1040,7 @@
 	PtSetResource(gui.vimPanelGroup, Pt_ARG_PG_PANEL_TITLES, &empty_title,
 		1);
 
-	vim_free(panel_titles);
-	panel_titles = NULL;
+	VIM_CLEAR(panel_titles);
     }
 }