Validate input UCI moves

Running following command:

  position startpos moves e1e8

Makes SF to assert in debug mode in do_move() but to accept
bad input and continue in release mode where probably it is
going to crash little later.

So validate input before to feed do_move().

Suggestion by Yakovlev Vadim.

No functional change.

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

src/position.cpp

@@ -777,6 +777,8 @@ bool Position::move_gives_check(Move m, const CheckInfo& ci) const {
 
 void Position::do_setup_move(Move m) {
 
+  assert(move_is_ok(m));
+
   StateInfo newSt;
 
   // Update the number of full moves after black's move