airo: sanitize handling of SSID_rid

* store SSID_rid without conversions
* sanitize proc_SSID_on_close() (and avoid access past the end of
  buffer, while we are at it)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>