check for material key validity in tbprobe

During recent work on threat inputs, there was a hard-to-debug crash in tbprobe
caused by incorrectly computing st->materialKey. This PR adds correctness
checking to pos_is_ok and a faster check in tbprobe that will actually run in
CI (because pos_is_ok checking is disabled even in debug mode, for performance
purposes).

closes https://github.com/official-stockfish/Stockfish/pull/6410

No functional change

src/position.h

@@ -169,6 +169,7 @@ class Position {
 
     // Position consistency check, for debugging
     bool pos_is_ok() const;
+    bool material_key_is_ok() const;
     void flip();
 
     StateInfo* state() const;
@@ -180,6 +181,7 @@ class Position {
    private:
     // Initialization helpers (used while setting up a position)
     void set_castling_right(Color c, Square rfrom);
+    Key  compute_material_key() const;
     void set_state() const;
     void set_check_info() const;