[3.15] gh-154176: Fix locale.strxfrm() crash on DragonFly BSD (GH-154177) (GH-154217)

Query the result size with a real one-element buffer instead of
wcsxfrm(NULL, s, 0): DragonFly BSD's wcsxfrm() crashes when the
destination is NULL or the size is 0.
(cherry picked from commit f389b03)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people

Misc/NEWS.d/next/Library/2026-07-19-21-30-00.gh-issue-154176.strxfrm.rst

@@ -0,0 +1,2 @@
+Fix a crash in :func:`locale.strxfrm` on DragonFly BSD,
+whose ``wcsxfrm()`` does not support the zero size.

Modules/_localemodule.c

@@ -443,6 +443,7 @@ _locale_strxfrm_impl(PyObject *module, PyObject *str)
 {
     Py_ssize_t n1;
     wchar_t *s = NULL, *buf = NULL;
+    wchar_t dummy[1];
     size_t n2;
     PyObject *result = NULL;
 
@@ -456,7 +457,9 @@ _locale_strxfrm_impl(PyObject *module, PyObject *str)
     }
 
     errno = 0;
-    n2 = wcsxfrm(NULL, s, 0);
+    /* Query the size with a real one-element buffer: DragonFly BSD's
+       wcsxfrm() crashes when the destination is NULL or the size is 0. */
+    n2 = wcsxfrm(dummy, s, 1);
     if (errno && errno != ERANGE) {
         PyErr_SetFromErrno(PyExc_OSError);
         goto exit;