qobject_to_qstring(obj) returns NULL when obj isn't a QString.  Check
that instead of qobject_type(obj) == QTYPE_QSTRING.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-10-git-send-email-armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>