Fix serialization of selected spellbook page

On game start, player interface attempts to load local state from empty
json, resetting all values to 0. However we want to show spellbook at
"all spells" page as default, not as top-most page with air spells

client/PlayerLocalState.cpp

@@ -396,16 +396,19 @@ void PlayerLocalState::deserialize(const JsonNode & source)
 		}
 	}
 
-	spellbookSettings.spellbookLastPageBattle = source["spellbook"]["pageBattle"].Integer();
-	spellbookSettings.spellbookLastPageAdvmap = source["spellbook"]["pageAdvmap"].Integer();
-	spellbookSettings.spellbookLastTabBattle = source["spellbook"]["tabBattle"].Integer();
-	spellbookSettings.spellbookLastTabAdvmap = source["spellbook"]["tabAdvmap"].Integer();
+	if (!source["spellbook"].isNull())
+	{
+		spellbookSettings.spellbookLastPageBattle = source["spellbook"]["pageBattle"].Integer();
+		spellbookSettings.spellbookLastPageAdvmap = source["spellbook"]["pageAdvmap"].Integer();
+		spellbookSettings.spellbookLastTabBattle = source["spellbook"]["tabBattle"].Integer();
+		spellbookSettings.spellbookLastTabAdvmap = source["spellbook"]["tabAdvmap"].Integer();
+	}
 
 	// append any owned heroes / towns that were not present in loaded state
 	wanderingHeroes.insert(wanderingHeroes.end(), oldHeroes.begin(), oldHeroes.end());
 	ownedTowns.insert(ownedTowns.end(), oldTowns.begin(), oldTowns.end());
 
-//FIXME: broken, anything that is selected in here will be overwritten on NewTurn pack
+//FIXME: broken, anything that is selected in here will be overwritten on PlayerStartsTurn pack
 //	ObjectInstanceID selectedObjectID(source["currentSelection"].Integer());
 //	const CGObjectInstance * objectPtr = owner.cb->getObjInstance(selectedObjectID);
 //	const CArmedInstance * armyPtr = dynamic_cast<const CArmedInstance*>(objectPtr);