xkb: ignore floating slave devices when updating from master (#81885)

Introduced in 45fb3a93. When a device is
enabled, the master's locked state is pushed to the slave. If the device is
floating, no master exists and we triggered a NULL-pointer dereference
in XkbPushLockedStateToSlaves.

X.Org Bug 81885 <http://bugs.freedesktop.org/show_bug.cgi?id=81885

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Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Keith Packard <keithp@keithp.com>