server: return NULL from device_open_file on failure to allocate fd

[Daniel Lehman]

> Does this patch address a real issue or is it just by looking at the
> source?
> &file->obj should not lead to an invalid memory access because its just
> a more convenient way to do a type conversion. If file == NULL, then
> &file->obj will also be NULL. Of course its a matter of taste what is
> easier to read.
> 

Just looking at source

Now that you mention it, I looked at the binary and I see what you mean.  ignore