xen.lowlevel.xl: Return None on empty domain name

Previously PyString_FromString(NULL) was called, which caused assertion
failure.

Signed-off-by: Marek Marczykowski <marmarek@mimuw.edu.pl>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/python/xen/lowlevel/xl/xl.c

@@ -409,13 +409,14 @@ static PyObject *pyxl_domid_to_name(XlObject *self, PyObject *args)
 {
     char *domname;
     int domid;
-    PyObject *ret;
+    PyObject *ret = Py_None;
 
     if ( !PyArg_ParseTuple(args, "i", &domid) )
         return NULL;
 
     domname = libxl_domid_to_name(self->ctx, domid);
-    ret = PyString_FromString(domname);
+    if (domname)
+        ret = PyString_FromString(domname);
     free(domname);
 
     return ret;