Fix use of an initialized SearchStack

Fix use of an initialized SearchStack

In RootMoveList c'tor we allocate a search stack and then
call directly qsearch.

There is called init_node() that clears all the fields of the search
stack array that refers to current ply but not the the killer moves.

The killer moves cleared correspond to ply+2.

In id_loop() this is not a problem because killer moves of
corresponding ply are cleared anyway few instructions later,
but in RootMoveList c'tor we leave them uninitialized.

This patch fixes this very old bug. It comes direclty
from Glaurung age.

Bug spotted by Valgrind.

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