gh-146196: Fix Undefined Behavior in _PyUnicodeWriter_WriteASCIIString() (#146201)

Avoid calling memcpy(data + writer->pos, NULL, 0)
which has an undefined behavior.

Co-authored-by: Victor Stinner <vstinner@python.org>
(cherry picked from commit cd10a2e)

Author: ashm-dev

Misc/NEWS.d/next/Core_and_Builtins/2026-03-20-13-55-14.gh-issue-146196.Zg70Kb.rst

@@ -0,0 +1,2 @@
+Fix potential Undefined Behavior in :c:func:`PyUnicodeWriter_WriteASCII` by
+adding a zero-length check. Patch by Shamil Abdulaev.

Objects/unicodeobject.c

@@ -14054,6 +14054,10 @@ _PyUnicodeWriter_WriteASCIIString(_PyUnicodeWriter *writer,
     if (len == -1)
         len = strlen(ascii);
 
+    if (len == 0) {
+        return 0;
+    }
+
     assert(ucs1lib_find_max_char((const Py_UCS1*)ascii, (const Py_UCS1*)ascii + len) < 128);
 
     if (writer->buffer == NULL && !writer->overallocate) {