tools/xenstored: Check NULL domain in domain_chk_quota()

With separate control and xenstore domains, the xenstore domain itself
is no longer privileged.  Local socket connections do not have
conn->domain assigned, but domain_chk_quota() will go ahead and
dereference it.  The !domid_is_unprivileged(conn->id) check usually
means the function would have already returned.

Bail out on !conn->domain to avoid faulting.  This adds a similar check
to that in domain_max_chk()

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

tools/xenstored/domain.c

@@ -1762,7 +1762,7 @@ static bool domain_chk_quota(struct connection *conn, unsigned int mem)
 	time_t now;
 	struct domain *domain;
 
-	if (!conn || !domid_is_unprivileged(conn->id) ||
+	if (!conn || !conn->domain || !domid_is_unprivileged(conn->id) ||
 	    conn->is_ignored)
 		return false;