TpChat: don't crash with broken TpProperties implementations

TpChat: don't crash with broken TpProperties implementations

I forgot to take the Telepathy.Properties interface out of Gabble when I
removed Telepathy.Properties support from it. So Empathy called
ListProperties() on it anyway: fine, you might think, except the
callback set priv->had_properties_list to TRUE even if the method
returned an error. empathy_tp_chat_set_property() and
empathy_tp_chat_get_property() assume that if priv->had_properties_list is
TRUE, priv->properties is not NULL.

While I hope that we can delete this code entirely pretty soon, getting
this crash fix in seems like a good idea.

Reviewed-by: Jonny Lamb <jonny.lamb@collabora.co.uk>