XGetFeedbackControl: skip over unknown feedback controls

If the server sends an unknown feedback control class we would allocate
the amount of *protocol* bytes for the client representation but then
later mess up the actual composition of the classes due to
Feedback->length never being initialized.

So while in theory it looks like this:
   |-------------- allocated size -----------------|
   [led feedback] [ ?unknown? ] [kbd feedback][....]
with unknown being the size of the class on the protocol,
in practice it looks like this:
   |-------------- allocated size -----------------|
   [led feedback] [ ?unknown?                ] [kbd feedback][...]
with unknown being the size of whatever was in the Feeback->length
pointed to at the time. The content of unknown is never initialized.

Fix this by making unknown classes disappear so the above becomes:
   |------- allocated size ---------|
   [led feedback][kbd feedback][....]

Closes #16

Part-of: <https://gitlab.freedesktop.org/xorg/lib/libxi/-/merge_requests/18>

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