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_gtk_x11.c b/src/gui_gtk_x11.c
index ad51be6..2f88c2e 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -3838,8 +3838,7 @@
 # endif
     }
 #endif
-    vim_free(gui_argv);
-    gui_argv = NULL;
+    VIM_CLEAR(gui_argv);
 
 #if GLIB_CHECK_VERSION(2,1,3)
     /* Set the human-readable application name */
@@ -4668,8 +4667,7 @@
 		y += hh - pixel_height;
 	    gtk_window_move(GTK_WINDOW(gui.mainwin), x, y);
 	}
-	vim_free(gui.geom);
-	gui.geom = NULL;
+	VIM_CLEAR(gui.geom);
 
 	/* From now until everyone's stopped trying to set the window hints
 	 * to their correct minimum values, stop them being set as we need