Make a function(ToTempSummon()) which can convert Creature to TempSummon with a C++ cast(reinterpret_cast) and with a type check(isSummon()).

This function is like ToPlayer(), ToTotem(), etc.

This commit is like ede831bdd6f0cff481acc33f269fa7f8c78befd4 / r7496:
http://code.google.com/p/trinitycore/source/detail?r=ede831bdd6f0cff481acc33f269fa7f8c78befd4
As a result, this commit has the advantages and the "disadvantages" too, like r7496.

Please try to be polite if this commit causes some crashes.

--HG--
branch : trunk

src/game/ObjectMgr.cpp

@@ -141,7 +141,7 @@ bool SpellClickInfo::IsFitToRequirements(Player const* player, Creature const *
     Unit const * summoner = NULL;
     // Check summoners for party
     if (clickNpc->isSummon())
-        summoner = ((TempSummon*)clickNpc)->GetSummoner();
+        summoner = clickNpc->ToTempSummon()->GetSummoner();
     if (!summoner)
         summoner = clickNpc;