tty: ipwireless: fix possible NULL dereference

The function alloc_ctrl_packet() can fail and return NULL. Incase it
fails print an error message and exit.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Reviewed-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>