Drop checks whether we can get the op-sector from mission()

Drop checks whether we can get the op-sector from mission()

getsect() can fail here only when the coordinates are invalid.  The
first check uses coordinates from a successful sarg_xy(), so they
can't be invalid.  The second check uses coordinates of an object to
be put on the mission.  If these are invalid, game state is corrupt,
and failing the mission command doesn't improve the situation a bit.