Split out an invariant in order to better check that newtd, when provided, must be on a runqueue.
Split out an invariant in order to better check that newtd, when
provided, must be on a runqueue.

Tested by:	Giovanni Trematerra
		<giovanni dot trematerra at gmail dot com>
MFC:		2 weeks
X-MFC:		r202889

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase