Core/Misc: Fix some static analysis issues

Fix some static analysis issues about uninitialized values. Most of them are false positives, always initialized before being accessed, while some of them are real issues spotted by valgrind too.

src/server/game/Maps/MapManager.cpp

@@ -43,6 +43,9 @@ MapManager::MapManager()
 {
     i_gridCleanUpDelay = sWorld->getIntConfig(CONFIG_INTERVAL_GRIDCLEAN);
     i_timer.SetInterval(sWorld->getIntConfig(CONFIG_INTERVAL_MAPUPDATE));
+    memset(i_GridStates, 0, sizeof(i_GridStates));
+    i_GridStateErrorCount = 0;
+    _nextInstanceId = 0;
 }
 
 MapManager::~MapManager() { }