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>