fix possible crash in xcbxv output plugin

A null pointer dereference happens if reading a xv port attribute
(which has been reported as readable) fails. This issue exists for
example with proprietary (and a bit buggy ...) ati drivers;
nevertheless it shouldn't cause a segmentation fault (the non-xcb
version simply stores an unitialised value).
This patches solves the issue in a clean way for both branches.

Fixes debian bug #428612 :-)

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