gh-159739: Preserve the normalization buffer on allocation failure

Save the original buffer before calling PyMem_Resize(), which overwrites
its first argument even on failure.

Backport to 3.15 and 3.14; the affected allocation spans multiple lines
on those branches.

Author: JelleZijlstra

Misc/NEWS.d/next/Library/2026-10-19-19-33-35.gh-issue-159739.b5fbca.rst

@@ -0,0 +1 @@
+Fix a memory leak when Unicode normalization cannot grow its temporary buffer.

Modules/unicodedata.c

@@ -746,7 +746,8 @@ nfd_nfkd(PyObject *self, PyObject *input, int k)
         }
 
         if (run_length > sortbuflen) {
-            Py_UCS4 *new_sortbuf = PyMem_Resize(sortbuf, Py_UCS4, run_length);
+            Py_UCS4 *new_sortbuf = sortbuf;
+            PyMem_Resize(new_sortbuf, Py_UCS4, run_length);
             if (new_sortbuf == NULL) {
                 PyErr_NoMemory();
                 PyMem_Free(sortbuf);