Fix an off-by-one bug in extract_pv()

Fix an off-by-one bug in extract_pv()

In case we reach ply == PLY_MAX we exit the function
writing

pv[PLY_MAX] = MOVE_NONE;

And because SearchStack is defined as:

struct SearchStack {
  Move pv[PLY_MAX];
  Move currentMove;
  .....

We end up with the unwanted assignment

SearchStack.currentMove = MOVE_NONE;

Fortunatly this is harmless because currentMove is not used where
extarct_pv() is called. But neverthless this is a bug that
needs to be fixed.

Thanks to Uri Blass for spotting out this.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>