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/keys.c

@@ -407,7 +407,7 @@ keys_action_nkey (enum key action, int keynum)
         return key->str;
       i++;
     }
-  return (char *)0;
+  return NULL;
 }
 
 char *
@@ -440,7 +440,7 @@ keys_format_label (char *key, int keylen)
   int i;
 
   if (keylen > BUFSIZ)
-    return (char *)0;
+    return NULL;
 
   bzero (fmtkey, sizeof (fmtkey));
   if (len == 0)