bpo-36398: Fix a possible crash in structseq_repr(). (GH-12492)

If the first PyUnicode_DecodeUTF8() call fails in structseq_repr(),
_PyUnicodeWriter_Dealloc() will be called on an uninitialized
_PyUnicodeWriter.

Misc/NEWS.d/next/Core and Builtins/2019-03-21-22-19-38.bpo-36398.B_jXGe.rst

+Fix a possible crash in ``structseq_repr()``.

Objects/structseq.c

@@ -176,7 +176,7 @@ structseq_repr(PyStructSequence *obj)
                                                strlen(typ->tp_name),
                                                NULL);
     if (type_name == NULL) {
-        goto error;
+        return NULL;
     }
 
     _PyUnicodeWriter_Init(&writer);