Quit without crashing if no driver is available
If the simulated driver is disabled, and no other driver can create a HMD, then monado crashes because estimate->maybe_head
is set to true in the legacy builder.
If the prober finds a "maybe device", then legacy_open_system
returns SUCCESSS
and create_system
does not check if the device is really there, then create_system_compositor
crashes on a null pointer.