Core/Database: Add additional assert

Add additional assert to assure that helgrind reports related to SQLQueryHolderTask::Execute() and WorldSession::ProcessQueryCallbacks() are false positives.
The only chance for WorldSession::ProcessQueryCallbacks() to access the SQL queries before they are completed is if m_result.ready() is true before the end of SQLQueryHolderTask::Execute() .

Helgrind false positive log:
 Possible data race during read of size 8 at 0x2BD641E0 by thread #7
 Locks held: none
    at : SQLQueryHolder::GetPreparedResult(unsigned long) (QueryHolder.cpp:107)
    by : Player::LoadFromDB(unsigned int, SQLQueryHolder*) (Player.cpp:16917)
    by : WorldSession::HandlePlayerLogin(LoginQueryHolder*) (CharacterHandler.cpp:807)
    by : WorldSession::ProcessQueryCallbacks() (WorldSession.cpp:1112)
    by : WorldSession::Update(unsigned int, PacketFilter&) (WorldSession.cpp:401)
    by : World::UpdateSessions(unsigned int) (World.cpp:2646)
    by : World::Update(unsigned int) (World.cpp:2003)
    by : WorldRunnable::run() (WorldRunnable.cpp:60)
    by : ACE_Based::Thread::ThreadTask(void*) (Threading.cpp:186)
    by : ACE_OS_Thread_Adapter::invoke() (in /usr/lib/libACE-6.0.3.so)
    by : mythread_wrapper (hg_intercepts.c:233)
    by : start_thread (pthread_create.c:311)

 This conflicts with a previous write of size 8 by thread #4
 Locks held: none
    at : SQLQueryHolder::SetPreparedResult(unsigned long, PreparedResultSet*) (QueryHolder.cpp:140)
    by : SQLQueryHolderTask::Execute() (QueryHolder.cpp:196)
    by : SQLOperation::call() (SQLOperation.h:65)
    by : DatabaseWorker::svc() (DatabaseWorker.cpp:45)
    by : ACE_Task_Base::svc_run(void*) (in /usr/lib/libACE-6.0.3.so)
    by : ACE_Thread_Adapter::invoke_i() (in /usr/lib/libACE-6.0.3.so)
    by : ACE_Thread_Adapter::invoke() (in /usr/lib/libACE-6.0.3.so)
    by : mythread_wrapper (hg_intercepts.c:233)

 Address 0x2BD641E0 is 16 bytes inside a block of size 792 alloc'd
    at : operator new(unsigned long) (vg_replace_malloc.c:319)
    by : __gnu_cxx::new_allocator<std::pair<SQLElementData, SQLResultSetUnion> >::allocate(unsigned long, void const*) (new_allocator.h:104)
    by : std::_Vector_base<std::pair<SQLElementData, SQLResultSetUnion>, std::allocator<std::pair<SQLElementData, SQLResultSetUnion> > >::_M_allocate(unsigned long) (in /home/jackpoz/trinity/bin/worldserver)
    by : std::vector<std::pair<SQLElementData, SQLResultSetUnion>, std::allocator<std::pair<SQLElementData, SQLResultSetUnion> > >::_M_fill_insert(__gnu_cxx::__normal_iterator<std::pair<SQLElementData, SQLResultSetUnion>*, std::vector<std::pair<SQLElementData, SQLResultSetUnion>, std::allocator<std::pair<SQLElementData, SQLResultSetUnion> > > >, unsigned long, std::pair<SQLElementData, SQLResultSetUnion> const&) (vector.tcc:483)
    by : std::vector<std::pair<SQLElementData, SQLResultSetUnion>, std::allocator<std::pair<SQLElementData, SQLResultSetUnion> > >::insert(__gnu_cxx::__normal_iterator<std::pair<SQLElementData, SQLResultSetUnion>*, std::vector<std::pair<SQLElementData, SQLResultSetUnion>, std::allocator<std::pair<SQLElementData, SQLResultSetUnion> > > >, unsigned long, std::pair<SQLElementData, SQLResultSetUnion> const&) (stl_vector.h:1024)
    by : std::vector<std::pair<SQLElementData, SQLResultSetUnion>, std::allocator<std::pair<SQLElementData, SQLResultSetUnion> > >::resize(unsigned long, std::pair<SQLElementData, SQLResultSetUnion>) (stl_vector.h:707)
    by : SQLQueryHolder::SetSize(unsigned long) (QueryHolder.cpp:167)
    by : LoginQueryHolder::Initialize() (CharacterHandler.cpp:66)
    by : WorldSession::HandlePlayerLoginOpcode(WorldPacket&) (CharacterHandler.cpp:788)
    by : WorldSession::Update(unsigned int, PacketFilter&) (WorldSession.cpp:363)
    by : World::UpdateSessions(unsigned int) (World.cpp:2646)
    by : World::Update(unsigned int) (World.cpp:2003)