bgpd: Fix crash when disabling dampening (BZ#687)

bgpd: Fix crash when disabling dampening (BZ#687)

    Vladimir Podobaev reported that the following commands crashed the
daemon.

router bgp 123
 bgp dampening
 no bgp dampening 1 2 3 4
 no bgp dampening

    The problem was that bgp_damp_info_clean() tried to dereference the
already freed reuse_list array in the second call to "no bgp dampening".

    Fixed by checking in bgp_damp_disable() that the dampening it's
enabled before doing the cleanup.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>