bgpd: Fix memory leak for `set as-path replace` route-map command

bgpd: Fix memory leak for `set as-path replace` route-map command

==1174993== 252 (120 direct, 132 indirect) bytes in 3 blocks are definitely lost in loss record 77 of 100
==1174993==    at 0x483AB65: calloc (vg_replace_malloc.c:760)
==1174993==    by 0x493C8D5: qcalloc (memory.c:116)
==1174993==    by 0x378E38: aspath_dup (bgp_aspath.c:698)
==1174993==    by 0x2A39E2: route_set_aspath_replace (bgp_routemap.c:2259)
==1174993==    by 0x4965C71: route_map_apply_ext (routemap.c:2664)
==1174993==    by 0x27BCC8: bgp_input_modifier (bgp_route.c:1657)
==1174993==    by 0x281AB9: bgp_update (bgp_route.c:3992)
==1174993==    by 0x286368: bgp_nlri_parse_ip (bgp_route.c:5890)
==1174993==    by 0x264D20: bgp_nlri_parse (bgp_packet.c:347)
==1174993==    by 0x2682FE: bgp_update_receive (bgp_packet.c:1921)
==1174993==    by 0x26AA67: bgp_process_packet (bgp_packet.c:2822)
==1174993==    by 0x498F5FD: thread_call (thread.c:2008)
==1174993==    by 0x49253DA: frr_run (libfrr.c:1198)
==1174993==    by 0x1EEC38: main (bgp_main.c:520)

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