bgpd: Free memory allocated by ecommunity_ecom2str()

bgpd: Free memory allocated by ecommunity_ecom2str()

Return local string, copied from returned value of ecom2str.

```
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690:Direct leak of 528 byte(s) in 8 object(s) allocated from:
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690-    0 0x7efcde5d6037 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690-    1 0x7efcde1dc7e2 in qcalloc lib/memory.c:105
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690-    2 0x5628a0592704 in ecommunity_ecom2str bgpd/bgp_ecommunity.c:947
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690-    3 0x7efcdaa558c8 in mplsL3vpnVrfRtTable bgpd/bgp_mplsvpn_snmp.c:1152
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690-    4 0x7efcda784139 in netsnmp_old_api_helper helpers/old_api.c:332
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690-
./bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn/r1.bgpd.asan.2925690-SUMMARY: AddressSanitizer: 528 byte(s) leaked in 8 allocation(s).
```

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