dirs: fix leak of iterator in dirs_fromiter stable

dirs: fix leak of iterator in dirs_fromiter

Spotted with cpychecker.

--- a/mercurial/dirs.c	Tue Jan 27 10:07:04 2015 -0500
+++ b/mercurial/dirs.c	Tue Jan 27 10:10:04 2015 -0500
@@ -175,6 +175,7 @@
 	}
 
 	ret = PyErr_Occurred() ? -1 : 0;
+	Py_DECREF(iter);
 	Py_XDECREF(item);
 	return ret;
 }