newlocale: fix crash when trying to write to __C_locale

newlocale: fix crash when trying to write to __C_locale

This simple testcase:

  locale_t st = newlocale(LC_ALL_MASK, "C", (locale_t)0);
  locale_t st2 = newlocale(LC_CTYPE_MASK, "en_US.UTF-8", st);

is sufficient to reproduce a crash in _newlocale_r.  After the first call
to newlocale, `st' points to __C_locale, which is const.  When using `st'
as locale base in the second call, _newlocale_r tries to set pointers
inside base to NULL.  This is bad if base is __C_locale, obviously.

Add a test to avoid trying to overwrite pointer values inside base if
base is __C_locale.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>