Fix a crash when reaching PLY_MAX in a check position

In this case we call evaluate() being in check and this
is not allowed.

Bug found testing with reduced PLY_MAX value as suggested
by Miguel A. Ballicora on talkchess.

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

src/evaluate.cpp

@@ -309,6 +309,7 @@ Value do_evaluate(const Position& pos, EvalInfo& ei, int threadID) {
 
   assert(pos.is_ok());
   assert(threadID >= 0 && threadID < THREAD_MAX);
+  assert(!pos.is_check());
 
   memset(&ei, 0, sizeof(EvalInfo));