dix: fix two crashes triggered by disabling master devices
Fixes crashers when
- a device is reattached to a pointer when the corresponding keyboard is still disabled
- removing a master pointer when the corresponding keyboard is still disabled
See #1611 (closed) for the reproducer
Fixes #1611 (closed)
Note: this only fixes the crashers, there's at least one use-case left that triggers the (hereby introduced) bug warning (disabling a master keyboard and then trying to remove it) but... meh. All this is niche enough as it is.