server: guest_set_client_capabilities: protect against NULL worker->display_channel

Reported-by: Michal Luscon <[email protected]>

Found by a Coverity scan:
  in handle_dev_start -
    Checking "worker->display_channel" implies that "worker->display_channel"
	         might be NULL.
    Passing "worker" to function "guest_set_client_capabilities"
  in guest_set_client_capabilities -
    Directly dereferencing parameter "worker->display_channel"

Author: uril

server/red_worker.c

@@ -10342,7 +10342,8 @@ static void guest_set_client_capabilities(RedWorker *worker)
         worker->set_client_capabilities_pending = 1;
         return;
     }
-    if (worker->display_channel->common.base.clients_num == 0) {
+    if ((worker->display_channel == NULL) ||
+        (worker->display_channel->common.base.clients_num == 0)) {
         worker->qxl->st->qif->set_client_capabilities(worker->qxl, FALSE, caps);
     } else {
         // Take least common denominator