Can't drag event to greyed-out days of the next or previous month

## What Happened
Dragging an event to the grey days of the next or previous months will redirect the event to the same day number of the current month. A more problematic side-effect is that, if you drop an event from a 30-day month to the 31st (grey) day of the previous month, the Calendar app crashes. What's worse is that it's no longer possible to open the Calendar app. It will open and immediately crash. A fix is to delete the event using Evolution.

## Expected Behavior
Ideally, the event should be moved to the actual day it is dragged onto.

## Steps to Reproduce
1. Create an event in September
2. Move it to 31 August
3. App crashes

### Logs
Crash with event dragged to the 31st day of the previous month:

```
Jul 25 13:03:02 Rossy gala[1844]: clutter_actor_remove_child: assertion 'CLUTTER_IS_ACTOR (child)' failed
Jul 25 13:03:02 Rossy gala[1844]: invalid (NULL) pointer instance
Jul 25 13:03:02 Rossy gala[1844]: g_signal_connect_data: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed
Jul 25 13:03:02 Rossy kernel: [ 288.463337] io.elementary.c[2538]: segfault at 10 ip 00007fee39c693c0 sp 00007fff949d6a50 error 4 in libglib-2.0.so.0.6400.6[7fee39c4b000+84000]
Jul 25 13:03:02 Rossy kernel: [ 288.463343] Code: 86 a0 06 00 48 8d 3d 6d 5c 06 00 e8 7a fb 01 00 31 c0 48 83 c4 08 c3 0f 1f 00 f3 0f 1e fa 55 48 89 f5 53 48 89 fb 48 83 ec 08 <8b> 77 10 48 8b 7f 08 e8 f4 66 04 00 48 63 73 14 48 89 ef 48 ba 00ap
```

## Platform Information

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