bpo-34910: Ensure that PyObject_Print() always returns -1 on error. (GH-9733)

(cherry picked from commit ae62f01)

Co-authored-by: Zackery Spytz <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/C API/2018-10-05-17-06-49.bpo-34910.tSFrls.rst

@@ -0,0 +1,2 @@
+Ensure that :c:func:`PyObject_Print` always returns ``-1`` on error.  Patch
+by Zackery Spytz.

Objects/object.c

@@ -387,8 +387,9 @@ PyObject_Print(PyObject *op, FILE *fp, int flags)
             else if (PyUnicode_Check(s)) {
                 PyObject *t;
                 t = PyUnicode_AsEncodedString(s, "utf-8", "backslashreplace");
-                if (t == NULL)
-                    ret = 0;
+                if (t == NULL) {
+                    ret = -1;
+                }
                 else {
                     fwrite(PyBytes_AS_STRING(t), 1,
                            PyBytes_GET_SIZE(t), fp);