gh-159739: Release replaced PyInitConfig strings (GH-159769)

* gh-159739: Release replaced PyInitConfig strings

Backport to 3.15 and 3.14.

---------

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: jelle-openai

Misc/NEWS.d/next/C_API/2026-10-19-20-38-17.gh-issue-159739.eb9cdc.rst

@@ -0,0 +1,2 @@
+:c:func:`PyInitConfig_SetStr` now releases a previously configured value and
+preserves it if conversion of the replacement fails.

Python/initconfig.c

@@ -4368,14 +4368,17 @@ PyInitConfig_SetStr(PyInitConfig *config, const char *name, const char* value)
 
     if (value == NULL && spec->type != PyConfig_MEMBER_WSTR_OPT) {
         initconfig_set_error(config, "config option string cannot be NULL");
+        return -1;
     }
 
     wchar_t **member = raw_member;
 
-    *member = utf8_to_wstr(config, value);
-    if (*member == NULL) {
+    wchar_t *wstr = utf8_to_wstr(config, value);
+    if (wstr == NULL) {
         return -1;
     }
+    free(*member);
+    *member = wstr;
     return 0;
 }