fix AV on switch from arcade to nonstop

stepmania/src/CourseContentsList.cpp

@@ -77,6 +77,8 @@ void CourseContentsList::SetFromGameState()
 		FOREACH_EnabledPlayer(pn)
 		{
 			Trail* pTrail = GAMESTATE->m_pCurTrail[pn];
+			if( pTrail == NULL )
+				continue;
 			pte[pn] = &pTrail->m_vEntries[i];
 		}
 		display.LoadFromTrailEntry( m_iNumContents+1, pte );