Xi: don't crash when deleting invalid device properties.

Deleting a property that was not set on a device leads to a null-pointer
reference. The protocol allows deleting those properties - it has to be a
noop.

Reproducible:
xinput --set-prop "My device" --type=int --format=8 "my property" 1
xinput --delete-prop "My other device" "my property"

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Keith Packard <keithp@keithp.com>

Xi/xiproperty.c

@@ -643,6 +643,9 @@ XIDeleteDeviceProperty (DeviceIntPtr device, Atom property, Bool fromClient)
         if (prop->propertyName == property)
             break;
 
+    if (!prop)
+        return Success;
+
     if (fromClient && !prop->deletable)
         return BadAccess;