(svn r26110) -Codechange: add guard for enterdir being valid in signal code

src/signal.cpp

@@ -294,6 +294,7 @@ static SigFlags ExploreSegment(Owner owner)
 					}
 				}
 
+				assert(IsValidDiagDirection(enterdir));
 				TrackBits tracks = GetTrackBits(tile); // trackbits of tile
 				TrackBits tracks_masked = (TrackBits)(tracks & _enterdir_to_trackbits[enterdir]); // only incidating trackbits