pythongh-146196: Fix potential Undefined Behavior in

_PyUnicodeWriter_WriteASCIIString

Author: ashm-dev

Objects/unicode_writer.c

@@ -465,6 +465,9 @@ _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) {
@@ -491,7 +494,7 @@ _PyUnicodeWriter_WriteASCIIString(_PyUnicodeWriter *writer,
         const Py_UCS1 *str = (const Py_UCS1 *)ascii;
         Py_UCS1 *data = writer->data;
 
-        memcpy(data + writer->pos, str, len);
+        memmove(data + writer->pos, str, len);
         break;
     }
     case PyUnicode_2BYTE_KIND: