Fix a crash introduced in 6ad4044f05 (I should be more attentive)

src/server/game/Entities/Player/Player.cpp

@@ -455,7 +455,7 @@ inline void KillRewarder::_InitGroupData()
                 }
         // 2.5. _isFullXP - flag identifying that for all group members victim is not gray,
         //      so 100% XP will be rewarded (50% otherwise).
-        _isFullXP = (_maxLevel == _maxNotGrayMember->getLevel());
+        _isFullXP = _maxNotGrayMember && (_maxLevel == _maxNotGrayMember->getLevel());
     }
     else
         _count = 1;