Core/Misc: Fix uninitialized values

Fix Position members not being always initialized.
Valgrind log:
 at : Position::NormalizeOrientation(float) (Object.h:388)
 by : Position::SetOrientation(float) (Object.h:315)
 by : Position::Relocate(Position const&) (Object.h:310)
 by : spell_ulduar_stone_grip::spell_ulduar_stone_grip_AuraScript::OnRemoveVehicle(AuraEffect const*, AuraEffectHandleModes) (boss_kologarn.cpp:592)