Fix the refleak in strftime when converting a %Z with a user defined tzinfo.

I inverted some of the conditionals to reduce indent levels.  Hopefully
this makes it a little easier to read.

This code caused the leak:

    class FixedOffset(datetime.tzinfo):
        def tzname(self, dt): return "UTC"

    datetime.time(12, 47, tzinfo=FixedOffset()).strftime('%Z')

This code is very tricky and I'm not positive it works.  However,
it neither crashes nor leaks.

Modules/datetimemodule.c

@@ -1131,45 +1131,52 @@ format_utcoffset(char *buf, size_t buflen, const char *sep,
 static PyObject *
 make_Zreplacement(PyObject *object, PyObject *tzinfoarg)
 {
+	PyObject *temp;
 	PyObject *tzinfo = get_tzinfo_member(object);
 	PyObject *Zreplacement = PyString_FromString("");
 	if (Zreplacement == NULL)
 		return NULL;
-	if (tzinfo != Py_None && tzinfo != NULL) {
-		PyObject *temp;
+	if (tzinfo == Py_None || tzinfo == NULL)
+		return Zreplacement;
+
 	assert(tzinfoarg != NULL);
 	temp = call_tzname(tzinfo, tzinfoarg);
 	if (temp == NULL)
 		goto Error;
-		if (temp != Py_None) {
+	if (temp == Py_None) {
+		Py_DECREF(temp);
+		return Zreplacement;
+	}
+
 	assert(PyUnicode_Check(temp));
 	/* Since the tzname is getting stuffed into the
 	 * format, we have to double any % signs so that
 	 * strftime doesn't treat them as format codes.
 	 */
 	Py_DECREF(Zreplacement);
-			Zreplacement = PyObject_CallMethod(temp, "replace",
-							   "ss", "%", "%%");
+	Zreplacement = PyObject_CallMethod(temp, "replace", "ss", "%", "%%");
 	Py_DECREF(temp);
 	if (Zreplacement == NULL)
 		return NULL;
 	if (PyUnicode_Check(Zreplacement)) {
-				Zreplacement =
-					_PyUnicode_AsDefaultEncodedString(
-						Zreplacement, NULL);
-				if (Zreplacement == NULL)
+		/* XXX(nnorwitz): this is really convoluted, is it correct? */
+		PyObject *Zreplacement2 =
+			_PyUnicode_AsDefaultEncodedString(Zreplacement, NULL);
+		if (Zreplacement2 == NULL)
 			return NULL;
-                                Py_INCREF(Zreplacement);
+		Py_INCREF(Zreplacement2);
+		/* Zreplacement is owned, but Zreplacement2 is borrowed.
+		   If they are different, we have to release Zreplacement. */
+		if (Zreplacement != Zreplacement2) {
+			Py_DECREF(Zreplacement);
+		}
+		Zreplacement = Zreplacement2;
 	}
 	if (!PyString_Check(Zreplacement)) {
 		PyErr_SetString(PyExc_TypeError,
 				"tzname.replace() did not return a string");
 		goto Error;
 	}
-		}
-		else
-			Py_DECREF(temp);
-	}
 	return Zreplacement;
 
   Error: