Fix use of uninitialized variable

When initializing endgames map we build a faked FEN string
in mat_key() to get the position hash's key.

This fen string lacks full move numbers, so when parsing the
fen in Position::from_fen() we leave startPosPly un-initialized.

Spotted by Valgrind (this is a kind of bug that is almost impossible
for humans to find).

No functional change.

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