reachableroots: return NULL if we're throwing an exception

Based on my reading of [0] and surrounding sections, if we want an
exception to be properly raised when something goes wrong in the C
code, we need to make sure we return NULL here. Do so.

https://docs.python.org/2/extending/extending.html#back-to-the-example

mercurial/parsers.c

@@ -1232,9 +1232,7 release_seen_and_tovisit:
 release_reachable:
 	Py_XDECREF(reachable);
 bail:
-	val = Py_None;
-	Py_INCREF(Py_None);
-	return val;
+	return NULL;
 }
 
 static PyObject *compute_phases_map_sets(indexObject *self, PyObject *args)