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>

newlib/libc/locale/newlocale.c

@@ -188,7 +188,8 @@ _newlocale_r (struct _reent *p, int category_mask, const char *locale,
 	if (tmp_locale.lc_cat[i].buf == (const void *) -1)
 	  {
 	    tmp_locale.lc_cat[i].buf = base->lc_cat[i].buf;
-	    base->lc_cat[i].ptr = base->lc_cat[i].buf = NULL;
+	    if (base != __get_C_locale ())
+	      base->lc_cat[i].ptr = base->lc_cat[i].buf = NULL;
 	  }
 #endif /* __HAVE_LOCALE_INFO__ */
       _freelocale_r (p, base);

winsup/cygwin/release/3.3.6

@@ -39,3 +39,6 @@ Bug Fixes
 - Fix a path handling bug that could cause a non-existing file to be
   treated as the current directory.
   Addresses: https://cygwin.com/pipermail/cygwin/2022-August/252030.html
+
+- Fix a crash in newlocale.
+  Addresses: https://cygwin.com/pipermail/cygwin/2022-August/252043.html