Prevent adding outgoing transition for top-most state

Prevent adding outgoing transition for top-most state

The outgoing transition for the top-most
state of the state stack in the DFSExplorer refers
to that which was taken as part of the last
trace explored by the algorithm. Thus, only the
sequence of transitions leading up to, but
not including, the last state must be included
when reconstructing the Exploration for SDPOR.