Don't IID when in check also in PvNodes

This tiny functional change allows to
nicely simplify things.

Performed at 50% in short TC:
LLR: -0.43 (-2.94,2.94)
Total: 46406 W: 9681 L: 9565 D: 27160

And succesfully passed long TC reverse test:
LLR: -2.95 (-2.94,2.94)
Total: 4945 W: 858 L: 937 D: 3150

bench: 4507230
Merge branch 'master' of git://github.com/mcostalba/Stockfish