fix of osmoload, flashing c155 works

[Sylvain Munaut]

>  - I'm not sure it's the correct way to fix it. Having mgb_pull return
> a pointer to the end of the bytes we just removed seems a very weird
> API ... maybe the bug is msgb_pull itself.

Ok, I guess changing the api of pull wouldn't be a great idea because
of other users and symmetry to _put ...

But then, I would introduce a msgb_pull_buffer() or _data  that
returns a pointer to the buffer/data zone that's just been pulled.

Cheers,

    Sylvain