[10042] Implement CONDITION_QUESTAVAILABLE.

Also do come constatification of quest status check functions
and fix one unsafe use std::map operator[].
Never never use std::map operator [] for _read_ access.
Good chance corrupt related structure state ;)
Not in found case but in general.

src/game/ObjectMgr.h

@@ -317,9 +317,10 @@ enum ConditionType
     CONDITION_NOITEM                = 16,                   // item_id      count
     CONDITION_SPELL                 = 17,                   // spell_id     0, 1 (0: has spell, 1: hasn't spell)
     CONDITION_INSTANCE_SCRIPT       = 18,                   // map_id       instance_condition_id (instance script specific enum)
+    CONDITION_QUESTAVAILABLE        = 19,                   // quest_id     0       for case when loot/gossip possible only if player can start quest
 };
 
-#define MAX_CONDITION                 19                    // maximum value in ConditionType enum
+#define MAX_CONDITION                 20                    // maximum value in ConditionType enum
 
 struct PlayerCondition
 {