bgpd: Fix memory leak for `as-override`

bgpd: Fix memory leak for `as-override`

==536197== 400 (160 direct, 240 indirect) bytes in 4 blocks are definitely lost in loss record 19 of 21
==536197==    at 0x483DD99: calloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==536197==    by 0x491C753: qcalloc (memory.c:116)
==536197==    by 0x303FA9: aspath_dup (bgp_aspath.c:698)
==536197==    by 0x304B2A: aspath_replace_specific_asn (bgp_aspath.c:1219)
==536197==    by 0x256840: bgp_peer_as_override (bgp_route.c:1781)
==536197==    by 0x256840: subgroup_announce_check (bgp_route.c:2216)
==536197==    by 0x258345: subgroup_process_announce_selected (bgp_route.c:2804)
==536197==    by 0x27F2CA: group_announce_route_walkcb (bgp_updgrp_adv.c:199)
==536197==    by 0x4905A51: hash_walk (hash.c:285)
==536197==    by 0x27E8D1: update_group_af_walk (bgp_updgrp.c:1866)
==536197==    by 0x2809D3: group_announce_route (bgp_updgrp_adv.c:1022)
==536197==    by 0x257DC4: bgp_process_main_one (bgp_route.c:3189)
==536197==    by 0x257DC4: bgp_process_main_one (bgp_route.c:2975)
==536197==    by 0x2581F7: bgp_process_wq (bgp_route.c:3330)
==536197==    by 0x4961787: work_queue_run (workqueue.c:285)
==536197==    by 0x4957745: thread_call (thread.c:2008)
==536197==    by 0x4910B77: frr_run (libfrr.c:1198)
==536197==    by 0x1ED6AC: main (bgp_main.c:520)

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>