ui/gtk: fix crash at startup when no console is available

This patch fixes a segfault at QEMU startup, introduced in a08156321ab9a7d2fed9ee77dbfeea2a61ffd153.
gd_vc_find_current() return NULL, which is dereferenced without checking it.

While at it, disable the whole 'View' menu if no console exists.

Reproducer: qemu-system-i386 -M none -nodefaults

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1483263585-8101-1-git-send-email-hpoussin@reactos.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
