Don't enable the device if the reachable count is zero

A logic bug was introduced in bd451cfe1512fa69eac35a60dbe6df17bfc39154 in which running `graph()` several times with zero reachable nodes had the effect of calling `device_enable()` (instead of keeping the device disabled).

This results in weird behavior when `DeviceStandby` is enabled, especially on Windows where calling `device_enable()` several times in a row corrupts I/O structures for the device, rendering it unusable.