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/utils.c

@@ -419,7 +419,7 @@ item_in_popup (char *saved_a_start, char *saved_a_end, char *msg,
   const int padl = winl - 2, padw = winw - margin_left;
 
   pad = newpad (padl, padw);
-  popup_win = popup (winl, winw, 1, 2, pop_title, (char *)0, 1);
+  popup_win = popup (winl, winw, 1, 2, pop_title, NULL, 1);
   if (strncmp (pop_title, _("Appointment"), 11) == 0)
     {
       mvwprintw (popup_win, margin_top, margin_left, "- %s -> %s",
@@ -557,7 +557,7 @@ new_tempfile (const char *prefix, int trailing_len)
           close (fd);
         }
       ERROR_MSG (_("temporary file \"%s\" could not be created"), fullname);
-      return (char *)0;
+      return NULL;
     }
   fclose (file);