Prevent sending empty line to engine after multi-leg move

Prevent sending empty line to engine after multi-leg move

As the pmoves are stored internally with a linefeed suffixed to them,
this linefeed was mistaken for a promotion character in the multi-leg
printing formats, and led to an extra empty line being sent to the
engine on non-promotion multi-leg moves. Now a linefeed is replaced
by a null character before sending the move.