Fix crash caused by FlameLeviathanPursuedTargetSelector

src/server/scripts/Northrend/Ulduar/ulduar/boss_flame_leviathan.cpp

@@ -1680,7 +1680,8 @@ class spell_pursue : public SpellScriptLoader
             void FilterTargetsSubsequently(std::list<Unit*>& targets)
             {
                 targets.clear();
-                targets.push_back(_target);
+                if(_target)
+                    targets.push_back(_target);
             }
 
             void HandleScript(SpellEffIndex /*eff*/)