vnc: Don't assert if opening unix socket fails

Reproducer:

$ qemu-system-x86_64 -display vnc=unix:/root/i-cant-access-you.sock
qemu-system-x86_64: iohandler.c:60: qemu_set_fd_handler2: Assertion `fd >= 0' failed.
Aborted (core dumped)

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

diff --git a/ui/vnc.c b/ui/vnc.c
index 076355d..fe4cd75 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -3684,6 +3684,9 @@
         /* listen for connects */
         if (strncmp(vnc, "unix:", 5) == 0) {
             vs->lsock = unix_listen(vnc+5, NULL, 0, errp);
+            if (vs->lsock < 0) {
+                goto fail;
+            }
             vs->is_unix = true;
         } else {
             vs->lsock = inet_listen_opts(sopts, 5900, errp);