lavapipe: Skip unbound targets in handle_end_transform_feedback().
It is legal to call vkCmdEndTransformFeedback() with valid counter buffers even where there is no corresponding XFB buffer bound.
llvmpipe handles this in vkCmdBeginTransformFeedback(), but currently crashes in vkCmdEndTransformFeedback().
--
This fixes a crash in Wine's test_index_buffer_offset(). We currently begin/end XFB with the maximum number of buffers, all valid, regardless of whether XFB is bound or not for any individual one.