Change type of track bits mask in MaskWireBits

This is to fix warnings from UndefinedBehaviorSanitizer.

src/elrail.cpp

@@ -146,7 +146,7 @@ static TrackBits MaskWireBits(TileIndex t, TrackBits tracks)
 	/* If the tracks from either a diagonal crossing or don't overlap, both
 	 * trackdirs have to be marked to mask the corresponding track bit. Else
 	 * one marked trackdir is enough the mask the track bit. */
-	TrackBits mask;
+	TrackBitsByte mask;
 	if (tracks == TRACK_BIT_CROSS || !TracksOverlap(tracks)) {
 		/* If the tracks form either a diagonal crossing or don't overlap, both
 		 * trackdirs have to be marked to mask the corresponding track bit. */