Fix a possible segfault. Found be Neal Norvitz.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index e24453d..2cb97bc 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -2890,7 +2890,7 @@
 	return -2;
     substr = PyUnicode_FromObject(substr);
     if (substr == NULL) {
-	Py_DECREF(substr);
+	Py_DECREF(str);
 	return -2;
     }