Issue #6697: use %U format instead of _PyUnicode_AsString(), because
_PyUnicode_AsString() was not checked for error (NULL).

The unicode string is no more truncated to 200 or 400 *bytes*.
diff --git a/Objects/funcobject.c b/Objects/funcobject.c
index 16851a9..35fc32d 100644
--- a/Objects/funcobject.c
+++ b/Objects/funcobject.c
@@ -295,9 +295,9 @@
 		    PyTuple_GET_SIZE(op->func_closure));
 	if (nclosure != nfree) {
 		PyErr_Format(PyExc_ValueError,
-			     "%s() requires a code object with %zd free vars,"
+			     "%U() requires a code object with %zd free vars,"
 			     " not %zd",
-			     _PyUnicode_AsString(op->func_name),
+			     op->func_name,
 			     nclosure, nfree);
 		return -1;
 	}