bgpd: fix no bgp as-path access-list issue

bgpd: fix no bgp as-path access-list  issue

router bgp 65001
 no bgp ebgp-requires-policy
 neighbor 192.168.1.2 remote-as external
 neighbor 192.168.1.2 timers 3 10
 address-family ipv4 unicast
  neighbor 192.168.1.2 route-map r2 in
 exit-address-family
!
ip prefix-list p1 seq 5 permit 172.16.255.31/32
!
route-map r2 permit 10
 match ip address prefix-list p1
 set as-path exclude 65003
route-map r2 permit 20
 set as-path exclude all
!

we make the following commands

bgp as-path access-list FIRST permit ^65
bgp as-path access-list SECOND permit 2
 route-map r2 permit 6
  set as-path exclude as-path-access-list SECOND

and then

no bgp as-path access-list SECOND permit 2
clear bgp *

we have the following crash in bgp

               Stack trace of thread 536083:
                #0  0x00007f87f8aacfe1 raise (libpthread.so.0 + 0x12fe1)
                #1  0x00007f87f8cf6870 core_handler (libfrr.so.0 +
		    0xf6870)
                #2  0x00007f87f8aad140 __restore_rt (libpthread.so.0 +
		    0x13140)
                #3  0x00007f87f89a5122 __GI___regexec (libc.so.6 +
		    0xdf122)
                #4  0x000055d7f198b4a7 aspath_filter_exclude_acl (bgpd +
		    0x2054a7)
                #5  0x000055d7f1902187 route_set_aspath_exclude (bgpd +
		    0x17c187)
                #6  0x00007f87f8ce54b0 route_map_apply_ext (libfrr.so.0
		    + 0xe54b0)
                #7  0x000055d7f18da925 bgp_input_modifier (bgpd +
		    0x154925)
                #8  0x000055d7f18e0647 bgp_update (bgpd + 0x15a647)
                #9  0x000055d7f18e4772 bgp_nlri_parse_ip (bgpd +
		    0x15e772)
                #10 0x000055d7f18c38ae bgp_nlri_parse (bgpd + 0x13d8ae)
                #11 0x000055d7f18c6b7a bgp_update_receive (bgpd +
		    0x140b7a)
                #12 0x000055d7f18c8ff3 bgp_process_packet (bgpd +
		    0x142ff3)
                #13 0x00007f87f8d0dce0 thread_call (libfrr.so.0 +
		    0x10dce0)
                #14 0x00007f87f8cacb28 frr_run (libfrr.so.0 + 0xacb28)
                #15 0x000055d7f18435da main (bgpd + 0xbd5da)
                #16 0x00007f87f88e9d0a __libc_start_main (libc.so.6 +
		    0x23d0a)
                #17 0x000055d7f18415fa _start (bgpd + 0xbb5fa)

analysis

crash is due to the fact that there were always a pointer from
as-path exclude to deleted as-path access list.

fix
we add a backpointer mechanism to manage the dependency beetween
as-path access-list  and aspath exclude.

Signed-off-by: Francois Dumontet <francois.dumontet@6wind.com>