bpo-40834: Fix truncate when sending str object with channel (GH-20555)

(cherry picked from commit 29c117202e386bad1d66ae336e2fefa1a1809ee0)

Co-authored-by: An Long <aisk@users.noreply.github.com>
diff --git a/Python/pystate.c b/Python/pystate.c
index 071b976..9beefa8 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1708,7 +1708,7 @@
     struct _shared_str_data *shared = PyMem_NEW(struct _shared_str_data, 1);
     shared->kind = PyUnicode_KIND(obj);
     shared->buffer = PyUnicode_DATA(obj);
-    shared->len = PyUnicode_GET_LENGTH(obj) - 1;
+    shared->len = PyUnicode_GET_LENGTH(obj);
     data->data = (void *)shared;
     Py_INCREF(obj);
     data->obj = obj;  // Will be "released" (decref'ed) when data released.