Fix: use stored randombg resettime for calculating when to do the next daily reset of random battlegrounds

Thanks to HP for pointing this out.

--HG--
branch : trunk

src/game/World.cpp

@@ -2582,7 +2582,7 @@ void World::InitRandomBGResetTime()
         nextDayResetTime += DAY;
 
     // normalize reset time
-    m_NextRandomBGReset = m_NextRandomBGReset < curTime ? nextDayResetTime - DAY : nextDayResetTime;
+    m_NextRandomBGReset = bgtime < curTime ? nextDayResetTime - DAY : nextDayResetTime;
 
     if (!bgtime)
         sWorld.setWorldState(WS_BG_DAILY_RESET_TIME, uint64(m_NextRandomBGReset));