bgpd: Free previously dup'ed aspath attribute for aggregate routes

bgpd: Free previously dup'ed aspath attribute for aggregate routes

Fixes memory leak:

```
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233:Direct leak of 80 byte(s) in 2 object(s) allocated f
rom:
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    0 0x7fcce641b037 in __interceptor_calloc ../../
../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    1 0x7fcce601f874 in qcalloc lib/memory.c:105
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    2 0x55a9594c805e in aspath_dup bgpd/bgp_aspath.c:688
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    3 0x55a9594cf55b in bgp_aggr_aspath_prepare bgpd/bgp_aspath.c:2164
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    4 0x7fcce5fbc95a in hash_iterate lib/hash.c:252
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    5 0x55a9594cf82f in bgp_compute_aggregate_aspath_val bgpd/bgp_aspath.c:2223
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    6 0x55a9594cf5db in bgp_compute_aggregate_aspath bgpd/bgp_aspath.c:2179
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    7 0x55a9592740fb in bgp_add_route_to_aggregate bgpd/bgp_route.c:8035
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    8 0x55a9592750ec in bgp_aggregate_increment bgpd/bgp_route.c:8234
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    9 0x55a959263174 in bgp_update bgpd/bgp_route.c:4797
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    10 0x55a9592696c9 in bgp_nlri_parse_ip bgpd/bgp_route.c:6070
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    11 0x55a959210649 in bgp_nlri_parse bgpd/bgp_packet.c:339
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    12 0x55a95921a3a1 in bgp_update_receive bgpd/bgp_packet.c:2023
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    13 0x55a95922159b in bgp_process_packet bgpd/bgp_packet.c:2933
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    14 0x7fcce60eaa50 in thread_call lib/thread.c:1991
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    15 0x7fcce5fe2e54 in frr_run lib/libfrr.c:1185
./bgp_local_asn_dot.test_bgp_local_asn_dot_agg/r3.bgpd.asan.653233-    16 0x55a9590d256d in main bgpd/bgp_main.c:505
```

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