Generate an error message when core tries to assign a script's ScriptName which is already assigned.

This means if that happened the core override the older script.
As a result, the earlier one doesn't work and can't deallocate so generates memory leak.

This commit also fixes memory leaks caused by that.

Valgrind log:

==4753== 354 (312 direct, 42 indirect) bytes in 1 blocks are definitely lost in loss record 1,194 of 1,218
==4753==    at 0x4C2626C: operator new(unsigned long) (vg_replace_malloc.c:230)
==4753==    by 0x1039D85: AddSC_go_scripts() (go_scripts.cpp:1015)
==4753==    by 0xDC196F: AddScripts() (ScriptLoader.cpp:534)
==4753==    by 0xC19943: ScriptMgr::ScriptsInit() (ScriptMgr.cpp:70)
==4753==    by 0xD1CCAE: World::SetInitialWorldSettings() (World.cpp:1608)
==4753==    by 0x9100A6: Master::Run() (Master.cpp:234)
==4753==    by 0x90F5D6: main (Main.cpp:146)

--HG--
branch : trunk

src/game/ScriptMgr.cpp

@@ -160,10 +160,36 @@ void Script::RegisterSelf()
 {
     int id = GetScriptId(Name.c_str());
     if (id)
+    {
+        // try to find the script in assigned scripts
+        bool IsExist = false;
+        for (uint16 i = 0; i < MAX_SCRIPTS; ++i)
+        {
+            if (m_scripts[i])
+            {
+                // if the assigned script's name and the new script's name is the same
+                if (m_scripts[i]->Name == Name)
+                {
+                    IsExist = true;
+                    break;
+                }
+            }
+        }
+
+        // if the script doesn't assigned -> assign it!
+        if (!IsExist)
         {
             m_scripts[id] = this;
             ++num_sc_scripts;
         }
+        // if the script is already assigned -> delete it!
+        else
+        {
+            // TODO: write a better error message than this one :)
+            error_log("ScriptName: '%s' already assigned with the same ScriptName, so the script can't work.", Name.c_str());
+            delete this;
+        }
+    }
     else
     {
         if (Name.find("example") == std::string::npos)