Fix a bug in insert_pv() where minimum depth is zero

We implicitly considered the minimum depth stored in TT
to be Depth(0), but because we store values in TT also in
qsearch() where depth is < 0, we need to use a negative
number as minimum depth.

Bug spotted by Joona Kiiski.

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