Fix en-passant parsing from fen string

Fix en-passant parsing from fen string

According to standard en-passant is recorded in fen string regardless
of whether there is a pawn in position to make an en passant capture.

Instead internally we set ep square only if the pawn can be captured.
So teach from_fen() to correctly handle this difference.

Bug reported and fixed by Justin Blanchard.

No functional change.

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