sixtracklib/common: fix problem preventing build of sixtracklib in release mode
sixtracklib/common: fix problem preventing build of sixtracklib in release mode

- remove variable only needed in debug mode for an SIXTRL_ASSERT test

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib