Add a check for the function which is wrapped in Python.

Many games include that check, but not all, and I got a Segfault in Python which took me some time to understand. The only "issue", is that now, in many times, we will have the check twice, but this should not have measurable impacts.

PiperOrigin-RevId: 277051089
Change-Id: I5e1fa39916593c3120628054d2bd2a8a112c29ed

open_spiel/spiel.h

@@ -400,6 +400,13 @@ class State {
 
   virtual std::vector<double> ObservationAsNormalizedVector(
       Player player) const {
+    // We add this player check, to prevent errors if the game implementation
+    // lacks that check (in particular as this function is the one used in
+    // Python). This can lead to doing this check twice.
+    // TODO(author2): Do we want to prevent executing this twice for games
+    // that implement it?
+    SPIEL_CHECK_GE(player, 0);
+    SPIEL_CHECK_LT(player, num_players_);
     std::vector<double> normalized_observation;
     ObservationAsNormalizedVector(player, &normalized_observation);
     return normalized_observation;