Catch exceptions when detecting SELinux status

When SELinux is disabled on RHEL6, exception occurs when reading /proc/self/attr/current. Due to the exception the "selinux" fact is later **missing**.

    # sestatus 
    SELinux status:                 disabled
    # facter selinux
    Could not retrieve selinux: Invalid argument - /proc/self/attr/current
    # cat /proc/self/attr/current
    cat: /proc/self/attr/current: Invalid argument

Following simple patch catches any exception on File read. "selinux" fact then contains value "false".