0-length messages bug

[mchalupa]

When we get message of length 4096, the last message may have size for example 40, but in the buffer there's only 12 bytes.

(Actually, isn't that a bug in wayland? Wayland sends always whole events... - anyway, without wldbg the client works, but running wldbg leads to crash)

How to reproduce: run wldbg -i shotwell (current as of writing) and drag an image (there's bug in wldbg that will lead to sending full-length buffers)