Don't need to assert for pos.is_ok() when position is constant

It's only necessary to do the checking at the end of every non-const
member (including the constructors and from_fen()) of class Position.
Once the post-condition of every modifier guarantees the class invariant,
we don't need to verify sanity of the position as preconditions for outside
callers such as movegen, search etc. For non-class types such as Move and
Square we still need to assert of course.

Suggested by Rein Halbersma.

No functional change.

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

src/position.h

@@ -89,9 +89,10 @@ struct StateInfo {
 
 class Position {
 
-  // No default or copy c'tor allowed, default c'tor will not be generated
-  // anyhow because of user-defined c'tors.
+  // No defaul, copy c'tor or assignment allowed, default c'tor will not be
+  // generated anyhow because of user-defined c'tors.
   Position(const Position&);
+  Position& operator=(const Position&);
 
 public:
   Position(const Position& pos, int threadID);