Core/AchievementMgr: fix integer underflow neglecting already completed realm firsts

Closes #19334

(cherry picked from commit 9cfa7c752e1d61b02acb852d2fd304a80d10b60a)

src/server/game/Achievements/AchievementMgr.cpp

@@ -3332,6 +3332,9 @@ bool AchievementGlobalMgr::IsRealmCompleted(AchievementEntry const* achievement)
     if (itr->second == std::chrono::system_clock::time_point::min())
         return false;
 
+    if (itr->second == std::chrono::system_clock::time_point::max())
+        return true;
+
     // Allow completing the realm first kill for entire minute after first person did it
     // it may allow more than one group to achieve it (highly unlikely)
     // but apparently this is how blizz handles it as well
@@ -3567,7 +3570,7 @@ void AchievementGlobalMgr::LoadCompletedAchievements()
             continue;
         }
         else if (achievement->Flags & (ACHIEVEMENT_FLAG_REALM_FIRST_REACH | ACHIEVEMENT_FLAG_REALM_FIRST_KILL | ACHIEVEMENT_FLAG_REALM_FIRST_GUILD))
-            _allCompletedAchievements[achievementId] = std::chrono::system_clock::time_point::max();;
+            _allCompletedAchievements[achievementId] = std::chrono::system_clock::time_point::max();
     } while (result->NextRow());
 
     TC_LOG_INFO("server.loading", ">> Loaded %lu realm first completed achievements in %u ms.", (unsigned long)_allCompletedAchievements.size(), GetMSTimeDiffToNow(oldMSTime));