bgpd: fix cleanup of dampening configuration

bgpd: fix cleanup of dampening configuration

The bgp_damp_config_clean() function was deallocating some arrays without
resetting the variables that represent their sizes. This was leading to
some crashes because other parts of the code iterate over these arrays
by looking at their corresponding sizes, which could be invalid.

Fixes the following segfaults (which only happen under certain
circumstances):
vtysh -c "configure terminal" -c "router bgp 1" -c "bgp dampening"
vtysh -c "configure terminal" -c "router bgp 1" -c "no bgp dampening"
vtysh -c "configure terminal" -c "router bgp 1" -c "no bgp dampening 45"
vtysh -c "" -c "clear ip bgp dampening"

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>