Don't crash on disco requests with no id=''
Don't crash on disco requests with no id=''

It's illegal to send <iq/> stanzas without an id='' (per the first rule
of IQ Semantics in XMPP Core
<http://xmpp.org/rfcs/rfc3920.html#stanzas-semantics-iq>), but neither
ejabberd nor Loudmouth filters them out before they reach Gabble. If
passed an IQ with no id='', lm_iq_message_make_result() returns NULL;
connection_iq_disco_cb() assumed that it returned a valid message, and
hence crashed.

Other places which call lm_iq_message_make_result() already check if the
result was NULL, so this is the only place that needed fixing.

Fixes: fd.o#26271
Reviewed-by: Sjoerd Simons <1371fd10dd05a897e5b55dc6ae4fbafa1f84c6ec@collabora.co.uk>

community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble