Core/Creature: fix integer overflow in Creature::Update leading to endless thrashing of characters database

Closes #19182

src/server/game/Entities/Creature/Creature.cpp

@@ -531,7 +531,18 @@ void Creature::Update(uint32 diff)
                     if (targetGuid == dbtableHighGuid) // if linking self, never respawn (check delayed to next day)
                         SetRespawnTime(DAY);
                     else
-                        m_respawnTime = (now > linkedRespawntime ? now : linkedRespawntime) + urand(5, MINUTE); // else copy time from master and add a little
+                    {
+                        // else copy time from master and add a little
+                        time_t baseRespawnTime = std::max(linkedRespawntime, now);
+                        time_t const offset = urand(5, MINUTE);
+
+                        // linked guid can be a boss, uses std::numeric_limits<time_t>::max to never respawn in that instance
+                        // we shall inherit it instead of adding and causing an overflow
+                        if (baseRespawnTime <= std::numeric_limits<time_t>::max() - offset)
+                            m_respawnTime = baseRespawnTime + offset;
+                        else
+                            m_respawnTime = std::numeric_limits<time_t>::max();
+                    }
                     SaveRespawnTime(); // also save to DB immediately
                 }
             }