Core/Misc: Ensure GetVictim() returns a valid reference when expected

Add a new method EnsureVictim() which asserts a valid not NULL reference will be returned to the caller. Use this whenever the GetVictim() would be dereferenced .

src/server/game/AI/ScriptedAI/ScriptedFollowerAI.cpp

@@ -74,7 +74,7 @@ bool FollowerAI::AssistPlayerInCombat(Unit* who)
         return false;
 
     //not a player
-    if (!who->GetVictim()->GetCharmerOrOwnerPlayerOrPlayerItself())
+    if (!who->EnsureVictim()->GetCharmerOrOwnerPlayerOrPlayerItself())
         return false;
 
     //never attack friendly