Issue #13612: Fix a buffer overflow in case of a multi-byte encoding.

This is a belated backport of f7b47fb30169; Patch by Serhiy Storchaka.
diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 379aa01..b9abcac 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -2427,6 +2427,8 @@
 
     if (PyUnicode_GET_SIZE(u) != 256) {
         Py_DECREF(u);
+        PyErr_SetString(PyExc_ValueError,
+                        "multi-byte encodings are not supported");
         return XML_STATUS_ERROR;
     }