Core/Commands: Fix NULL dereference crash in .npc set link

Fix NULL dereference crash in .npc set link happening when passing a not-existent Creature guid as parameter, es. ".npc set link -1" .
Issue spotted by static analysis, added in c05ed659cb

src/server/game/Globals/ObjectMgr.cpp

@@ -1412,6 +1412,11 @@ bool ObjectMgr::SetCreatureLinkedRespawn(uint32 guidLow, uint32 linkedGuidLow)
     }
 
     const CreatureData* slave = GetCreatureData(linkedGuidLow);
+    if (!slave)
+    {
+        TC_LOG_ERROR("sql.sql", "Creature '%u' linking to non-existent creature '%u'.", guidLow, linkedGuidLow);
+        return false;
+    }
 
     const MapEntry* const map = sMapStore.LookupEntry(master->mapid);
     if (!map || !map->Instanceable() || (master->mapid != slave->mapid))