Xi: when creating a new ButtonClass, set the number of buttons

There's a racy sequence where a master device may copy the button class
from the slave, without ever initializing numButtons. This leads to a
device with zero buttons but a button class which is invalid.

Let's copy the numButtons value from the source - by definition if we
don't have a button class yet we do not have any other slave devices
with more than this number of buttons anyway.

CVE-2024-0229, ZDI-CAN-22678

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

Xi/exevents.c

@@ -605,6 +605,7 @@ DeepCopyPointerClasses(DeviceIntPtr from, DeviceIntPtr to)
                 to->button = calloc(1, sizeof(ButtonClassRec));
                 if (!to->button)
                     FatalError("[Xi] no memory for class shift.\n");
+                to->button->numButtons = from->button->numButtons;
             }
             else
                 classes->button = NULL;