Misc. compile warnings

[Bart Schaefer <schaefer@brasslantern.com>]

Just upgraded Mavericks and am seeing some compile warnings I don't recall
having encountered before:

compctl.c:3419:11: warning: expression which evaluates to zero treated as a null
      pointer constant of type 'char *' [-Wnon-literal-null-conversion]
                        *npp = '\0';
                               ^~~~

zle_refresh.c:432:13: warning: expression which evaluates to zero treated as a
      null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
    *arrp = '\0';
            ^~~~
zle_refresh.c:261:30: warning: unused variable 'zr_ht' [-Wunused-const-variable]
static const REFRESH_ELEMENT zr_ht = { ZWC('\t'), 0 };