Check container existance during drag events

This fixes a crash that occurs when disabling floating for a container
while it is being moved or resized.

@Deiz describes the problem:
> It occurs because the command that disables floating runs before the
event loop. So, the window is tiled, its floating parent is destroyed,
but then a key event is handled which causes the position/size of the
now-destroyed parent to be modified.

Fixes #1627