vnc: deal with surface NULL pointers

Secondary displays in multihead setups are allowed to have a NULL
DisplaySurface.  Typically user interfaces handle this by hiding the
window which shows the display in question.

This isn't an option for vnc though because it simply hasn't a concept
of windows or outputs.  So handle the situation by showing a placeholder
DisplaySurface instead.  Also check in console_select whenever a surface
is preset in the first place before requesting an update.

This fixes a segfault which can be triggered by switching to an unused
display (via vtrl-alt-<nr>) in a multihead setup, for example using
-device virtio-vga,max_outputs=2.

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-id: 20180308161803.6152-1-kraxel@redhat.com