Fix incorrect assert(PvNode == (alpha != beta - 1))

Fix incorrect assert(PvNode == (alpha != beta - 1))

In case of a PvNode could happen that alpha == beta - 1,
for instance in case the same previous node was visited
with same beta during a non-pv search, the node failed low
and stored beta-1 in TT. Then the node is searched again
in PV mode, TT value beta-1 is retrieved and updates alpha
that now happens to be beta-1.

No functional change.

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