dirs: fix leak of iterator in dirs_fromiter

Spotted with cpychecker.

mercurial/dirs.c

@@ -175,6 +175,7 b' static int dirs_fromiter(PyObject *dirs,'
 	}
 
 	ret = PyErr_Occurred() ? -1 : 0;
+	Py_DECREF(iter);
 	Py_XDECREF(item);
 	return ret;
 }