console: Fix segfault on screendump without VGA adapter

When trying to create a screen dump without having any VGA adapter
inside the guest, QEMU segfaults.

This is because it's trying to switch back to the "previous" screen
it was on before dumping the VGA screen. Unfortunately, in my case
there simply is no previous screen so it accesses a NULL pointer.

Fix it by checking if previous_active_console is actually available.

This is 1.0 material.

Signed-off-by: Alexander Graf <agraf@suse.de>
