Fix segmentation fault: vpninfo->ifname is NULL

The intent in 48bd28aa was to pass the ifname argument of create_ifname_w() to buf_append_utf16le(), not vpninfo->ifname.

Indeed vpninfo->ifname has not yet been initialised here. It is still NULL.

Passing a NULL argument to buf_append_utf16le() would cause a segmentation fault before 4d22f1de.

Fixes #370 (closed). I checked that on Windows 10 Professional 21H2.

Would probably have fixed #358 (closed) too.

Might fix !294 (closed) as well – to be verified.