Core/Player: Fix uninitialized Player field.

m_MonthlyQuestChanged was initialized only when loading a Player from DB and left uninitialized when creating a new Player.

Valgrind log:
 Conditional jump or move depends on uninitialised value(s)
   at 0x1148E2A: Player::_SaveMonthlyQuestStatus(Trinity::AutoPtr<Transaction, ACE_Thread_Mutex>&) (Player.cpp:19694)
   by 0x1146510: Player::SaveToDB(bool) (Player.cpp:19191)
   by 0x14F5D5C: WorldSession::HandleCharCreateCallback(Trinity::AutoPtr<PreparedResultSet, ACE_Thread_Mutex>, CharacterCreateInfo*) (CharacterHandler.cpp:660)

Closes #10620

Signed-off-by: Nay <dnpd.dd@gmail.com>

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

@@ -864,6 +864,8 @@ Player::Player(WorldSession* session): Unit(true)
 
     m_WeeklyQuestChanged = false;
 
+    m_MonthlyQuestChanged = false;
+
     m_SeasonalQuestChanged = false;
 
     SetPendingBind(0, 0);