Core/Movement: Fix invalid memory access

Fix the stack implementation used in MotionMaster and added few sanity checks to ensure no underflows will be made.

Valgrind log:
 Invalid read of size 8
  at : MotionMaster::top() const (MotionMaster.h:115)
  by : MotionMaster::pop() (MotionMaster.h:91)
  by : MotionMaster::~MotionMaster() (MotionMaster.cpp:74)
  by : Unit::~Unit() (Unit.cpp:296)
  by : Player::~Player() (Player.cpp:880)
  by : WorldSession::HandleCharCreateCallback(Trinity::AutoPtr<PreparedResultSet, ACE_Thread_Mutex>, CharacterCreateInfo*) (CharacterHandler.cpp:665)
  by : WorldSession::HandleCharCreateCallback(Trinity::AutoPtr<PreparedResultSet, ACE_Thread_Mutex>, CharacterCreateInfo*) (CharacterHandler.cpp:516)