python: fix qubesdb module in vm

The function qdb_init() should get NULL but instead it got uninitalised
local variable, since PyArg_ParseTuple does not touch pointers to
omitted argumets.

QubesOS/qubes-issues#1637

python/qdb.c

@@ -450,7 +450,7 @@ qdbhandle_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 static int
 qdbhandle_init(QdbHandle *self, PyObject *args)
 {
-    char *vmname;
+    char *vmname = NULL;
 
     if (!PyArg_ParseTuple(args, "|s", &vmname))
         goto fail;