Fix 'use after free' when deleting battlegrounds.

BattleGroundMap::Update() is calling Update() on the BattleGround
associated with the map, which is typically the Update() call of a
derived class like BattleGroundAB. These start by calling the base
function BattleGround::Update(). So when BattleGround::Update() decides
to delete the battleground and return, this is kind of sawing off the
branch we're still sitting on! If the object gets deleted, the Update()
call of the derived class can't doing anything any more, but in practice
it was happily continuing with its own Update() code, leading to all
kinds of nasty things (and usually a crash).

src/game/BattleGround/BattleGround.cpp

@@ -285,24 +285,6 @@ BattleGround::~BattleGround()
 */
 void BattleGround::Update(uint32 diff)
 {
-    if (!GetPlayersSize())
-    {
-        // BG is empty
-        // if there are no players invited, delete BG
-        // this will delete arena or bg object, where any player entered
-        // [[   but if you use battleground object again (more battles possible to be played on 1 instance)
-        //      then this condition should be removed and code:
-        //      if (!GetInvitedCount(HORDE) && !GetInvitedCount(ALLIANCE))
-        //          this->AddToFreeBGObjectsQueue(); // not yet implemented
-        //      should be used instead of current
-        // ]]
-        // BattleGround Template instance cannot be updated, because it would be deleted
-        if (!GetInvitedCount(HORDE) && !GetInvitedCount(ALLIANCE))
-            delete this;
-
-        return;
-    }
-
     // remove offline players from bg after 5 minutes
     if (!m_offlineQueue.empty())
     {

src/game/Maps/Map.cpp

@@ -1887,7 +1887,23 @@ void BattleGroundMap::Update(const uint32& diff)
 {
     Map::Update(diff);
 
-    GetBG()->Update(diff);
+    if (!m_bg->GetPlayersSize())
+    {
+        // BG is empty
+        // if there are no players invited, delete BG
+        // this will delete arena or bg object, where any player entered
+        // [[   but if you use battleground object again (more battles possible to be played on 1 instance)
+        //      then this condition should be removed and code:
+        //      if (!GetInvitedCount(HORDE) && !GetInvitedCount(ALLIANCE))
+        //          this->AddToFreeBGObjectsQueue(); // not yet implemented
+        //      should be used instead of current
+        // ]]
+        // BattleGround Template instance cannot be updated, because it would be deleted
+        if (!m_bg->GetInvitedCount(HORDE) && !m_bg->GetInvitedCount(ALLIANCE))
+            delete m_bg;
+    }
+    else
+        m_bg->Update(diff);
 }
 
 BattleGroundPersistentState* BattleGroundMap::GetPersistanceState() const