parsers: avoid leaking obj in index_ancestors stable

parsers: avoid leaking obj in index_ancestors

PySequence_GetItem returns a new reference. Found with cpychecker.

--- a/mercurial/parsers.c	Fri Jan 23 15:30:21 2015 -0500
+++ b/mercurial/parsers.c	Fri Jan 23 15:33:27 2015 -0500
@@ -1691,9 +1691,11 @@
 		if (!PyInt_Check(obj)) {
 			PyErr_SetString(PyExc_TypeError,
 					"arguments must all be ints");
+			Py_DECREF(obj);
 			goto bail;
 		}
 		val = PyInt_AsLong(obj);
+		Py_DECREF(obj);
 		if (val == -1) {
 			ret = PyList_New(0);
 			goto done;