ROOMBA_TASKS: Fixed sometimes ocurring crash in calcPath

- calcPath function didn't always find startV thus leaving it NULL,
  which causes a crash when referencing. This occured when Xloc_
  or Yloc_ was very near the borders of a vertice.
- floating point arithmetic seems to be EVIL so Xloc_ and Yloc_ are
  casted like this: unsigned(floor(Xloc_)) to represent their
  position in an integer world comparable to vertices locations