Return from win32_setlocale() needs to be stable

Prior to this commit it was subject to freeing when the scope was
exited.

This commit repurposes the already-existing per-thread buffer that holds
the return of setlocale(), stable until the next call to it.  Instead of
being used for setlocale() which isn't called directly on Windows
platforms, it uses it for win32_setlocale().

locale.c

@@ -3092,7 +3092,10 @@ S_find_locale_from_environment(pTHX_ const unsigned int index)
 #    ifdef WIN32
                 /* If no LANG, use the system default on Windows. */
                 locale_names[j] = wrap_wsetlocale(categories[i], ".ACP");
-                if (! locale_names[j])
+                if (locale_names[j]) {
+                    SAVEFREEPV(locale_names[j]);
+                }
+                else
 #    endif
                 {   /* If nothing was found or worked, use C */
                     locale_names[j] = "C";
@@ -4054,6 +4057,8 @@ S_wrap_wsetlocale(pTHX_ const int category, const char *locale)
 
     /* Calls _wsetlocale(), converting the parameters/return to/from
      * Perl-expected forms as if plain setlocale() were being called instead.
+     *
+     * Caller must arrange for the returned PV to be freed.
      */
 
     const wchar_t * wlocale = NULL;
@@ -4078,9 +4083,6 @@ S_wrap_wsetlocale(pTHX_ const int category, const char *locale)
     WSETLOCALE_UNLOCK;
 
     Safefree(wlocale);
-    SAVEFREEPV(result); /* is there something better we can do here?  Answer:
-                           Without restructuring, returning a unique value each
-                           call is required.  See GH #20434 */
     return result;
 }
 
@@ -4116,23 +4118,38 @@ S_win32_setlocale(pTHX_ int category, const char* locale)
         return NULL;
     }
 
-#    ifdef USE_PL_CUR_LC_ALL
+    save_to_buffer(result, &PL_setlocale_buf, &PL_setlocale_bufsize);
+
+#    ifndef USE_PL_CUR_LC_ALL
+
+    Safefree(result);
+
+#  else
+
+    /* Here, we need to keep track of LC_ALL, so store the new value.  but if
+     * the input locale is NULL, we were just querying, so the original value
+     * hasn't changed */
+    if (locale == NULL) {
+        Safefree(result);
+    }
+    else {
 
-    /* Here, we need to keep track of LC_ALL.  If we set it directly above, we
-     * already know what it is; otherwise query the new value. */
-    const char * new_lc_all = ((category == LC_ALL)
-                               ? result
-                               : wrap_wsetlocale(LC_ALL, NULL));
+        /* If we set LC_ALL directly above, we already know its new value; but
+         * if we changed just an individual category, find the new LC_ALL */
+        if (category != LC_ALL) {
+            Safefree(result);
+            result = wrap_wsetlocale(LC_ALL, NULL);
+        }
 
-    /* And update if it has changed. */
-    if (strNE(new_lc_all, PL_cur_LC_ALL)) {
         Safefree(PL_cur_LC_ALL);
-        PL_cur_LC_ALL = savepv(new_lc_all);
+        PL_cur_LC_ALL = result;
     }
 
+    DEBUG_L(PerlIO_printf(Perl_debug_log, "new PL_cur_LC_ALL=%s\n",
+                                          PL_cur_LC_ALL));
 #    endif
 
-    return result;
+    return PL_setlocale_buf;
 }
 
 #  endif