the pseudo-random routine pseudo-randomly generated X,Y for new piece of food
that corresponded to immediately next snake head position.
This X,Y resulted empty when checked in the routine, but suddenly after it
is overwritten with snake head, so food simply disappears.
This leads to impossible game play, because no new pieces of food are
generated if no food is previously eaten, but there is no food to eat, so...
Hope to have fixed.
