'errno' not reset prior to create_uio_info()

[bear101]

https://github.com/Linutronix/libuio/blob/ca28ff0f69d89a789a47552c72db5a43d280710b/base.c#L288

If 'errno' is already active when calling uio_find_by_uio_num() then libuio will think it's an error caused by libuio itself. Why not check for NULL instead of 'errno'?