Core/Calendar: Fix issue with invite status being set to the wrong invite

Consider the case of Player A inviting Player B to a Calendar Event. After Player B clicks on Accepts/Tentative/Reject, the calendar shows that status for Player A instead.

(cherry picked from commit 7f3d8c99bb14c7b093a5c2419f3e931fef11004e)

src/server/game/Calendar/CalendarMgr.cpp

@@ -32,7 +32,9 @@ _status(CALENDAR_STATUS_INVITED), _rank(CALENDAR_RANK_PLAYER), _note("") { }
 
 CalendarInvite::~CalendarInvite()
 {
-    sCalendarMgr->FreeInviteId(_inviteId);
+    // Free _inviteId only if it's a real invite and not just a pre-invite or guild announcement
+    if (_inviteId != 0 && _eventId != 0)
+        sCalendarMgr->FreeInviteId(_inviteId);
 }
 
 CalendarEvent::~CalendarEvent()