Make use of the NULL macro

Use this constant everywhere when referring to a null pointer instead of
casting 0 to various types of pointers. Created using following semantic
patch:

    @@
    type type;
    @@

    - (type *)0
    + NULL

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/custom.c

@@ -572,7 +572,7 @@ custom_layout_config (void)
       "       'a' -> appointment panel\n\n"
       "       't' -> todo panel\n\n");
 
-  conf_win.p = (WINDOW *)0;
+  conf_win.p = NULL;
   strncpy (label, _("layout configuration"), BUFSIZ);
   custom_confwin_init (&conf_win, label);
   cursor = mark = wins_layout () - 1;