locale.c: Use out-of-bounds value for illegality

This was using 0 as a parameter, which is a legal value and could have
been confused by the called subroutine as being legitimate.  It is not a
bug because of the way the subroutine is currently written given the
other parameters in this call.  But it is more robust to use an
out-of-bounds value, so that if the function were to change, we would
sooner find out about any improper call.

locale.c

@@ -5744,7 +5744,7 @@ S_my_localeconv(pTHX_ const int item)
             if (UTF8NESS_YES == (get_locale_string_utf8ness_i(SvPVX(*value),
                                                   LOCALE_IS_UTF8,
                                                   NULL,
-                                                  (locale_category_index) 0)))
+                                                  LC_ALL_INDEX_ /* OOB */)))
             {
                 SvUTF8_on(*value);
             }