Changes

Only check for ullage on ModuleEnginesRF

When trying to check for ullage on a non-ModuleEnginesRF engine,
VesselState sets the reflection fields to null; this causes
the autostaging code to spam NREs and refuse to work.

This change makes sure that an engine is a (subclass of) ModuleEnginesRF
before trying to access the ullage field.
 (commit: b54372b)