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)

src/server/game/Movement/MotionMaster.cpp

@@ -130,6 +130,9 @@ void MotionMaster::DirectClean(bool reset)
         if (curr) DirectDelete(curr);
     }
 
+    if (empty())
+        return;
+
     if (needInitTop())
         InitTop();
     else if (reset)
@@ -156,7 +159,7 @@ void MotionMaster::DirectExpire(bool reset)
         DirectDelete(curr);
     }
 
-    while (!top())
+    while (!empty() && !top())
         --_top;
 
     if (empty())
@@ -176,7 +179,7 @@ void MotionMaster::DelayedExpire()
         DelayedDelete(curr);
     }
 
-    while (!top())
+    while (!empty() && !top())
         --_top;
 }
 

src/server/game/Movement/MotionMaster.h

@@ -87,13 +87,21 @@ class MotionMaster //: private std::stack<MovementGenerator *>
 
         void pop()
         {
+            if (empty())
+                return;
+
             Impl[_top] = NULL;
-            while (!top())
+            while (!empty() && !top())
                 --_top;
         }
         void push(_Ty _Val) { ++_top; Impl[_top] = _Val; }
 
-        bool needInitTop() const { return _needInit[_top]; }
+        bool needInitTop() const 
+        { 
+            if (empty())
+                return false;
+            return _needInit[_top]; 
+        }
         void InitTop();
     public:
 
@@ -112,8 +120,16 @@ class MotionMaster //: private std::stack<MovementGenerator *>
 
         bool empty() const { return (_top < 0); }
         int size() const { return _top + 1; }
-        _Ty top() const { return Impl[_top]; }
-        _Ty GetMotionSlot(int slot) const { return Impl[slot]; }
+        _Ty top() const 
+        { 
+            ASSERT(!empty());
+            return Impl[_top]; 
+        }
+        _Ty GetMotionSlot(int slot) const 
+        { 
+            ASSERT(slot >= 0);
+            return Impl[slot]; 
+        }
 
         void DirectDelete(_Ty curr);
         void DelayedDelete(_Ty curr);