sd-network: fix memleak in dhcp6_option_parse_domainname (#5114)

sd-network: fix memleak in dhcp6_option_parse_domainname (#5114)

The simplest way to reproduce:
```diff
diff --git a/src/libsystemd-network/test-dhcp6-client.c b/src/libsystemd-network/test-dhcp6-client.c
index bd289fa..7b0a5ef 100644
--- a/src/libsystemd-network/test-dhcp6-client.c
+++ b/src/libsystemd-network/test-dhcp6-client.c
@@ -168,7 +168,7 @@ static uint8_t msg_advertise[198] = {
         0x00, 0x17, 0x00, 0x10, 0x20, 0x01, 0x0d, 0xb8,
         0xde, 0xad, 0xbe, 0xef, 0x00, 0x00, 0x00, 0x00,
         0x00, 0x00, 0x00, 0x01, 0x00, 0x18, 0x00, 0x0b,
-        0x03, 0x6c, 0x61, 0x62, 0x05, 0x69, 0x6e, 0x74,
+        0x01, 0x6c, 0x01, 0x62, 0x00, 0x0a, 0x6e, 0x74,
         0x72, 0x61, 0x00, 0x00, 0x1f, 0x00, 0x10, 0x20,
         0x01, 0x0d, 0xb8, 0xde, 0xad, 0xbe, 0xef, 0x00,
         0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00,
@@ -338,9 +338,7 @@ static int test_advertise_option(sd_event *e) {
         assert_se(!memcmp(addrs, &msg_advertise[124], r * 16));

         r = sd_dhcp6_lease_get_domains(lease, &domains);
-        assert_se(r == 1);
-        assert_se(!strcmp("lab.intra", domains[0]));
-        assert_se(domains[1] == NULL);
+        assert_se(r == -ENOENT);

         r = sd_dhcp6_lease_get_ntp_addrs(lease, &addrs);
         assert_se(r == 1);
```

Fixes:
```
=================================================================
==15043==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 4 byte(s) in 1 object(s) allocated from:
    #0 0x7f13c8564160 in strdup (/lib64/libasan.so.3+0x5a160)
    #1 0x7f13c7caaf69 in strv_extend src/basic/strv.c:552
    #2 0x55f775787230 in dhcp6_option_parse_domainname src/libsystemd-network/dhcp6-option.c:399
    #3 0x55f775788b96 in dhcp6_lease_set_domains src/libsystemd-network/sd-dhcp6-lease.c:225
    #4 0x55f775774b95 in test_advertise_option src/libsystemd-network/test-dhcp6-client.c:287
    #5 0x55f77577883e in main src/libsystemd-network/test-dhcp6-client.c:759
    #6 0x7f13c7589400 in __libc_start_main (/lib64/libc.so.6+0x20400)

Direct leak of 4 byte(s) in 1 object(s) allocated from:
    #0 0x7f13c8564160 in strdup (/lib64/libasan.so.3+0x5a160)
    #1 0x7f13c7caaf69 in strv_extend src/basic/strv.c:552
    #2 0x55f775787230 in dhcp6_option_parse_domainname src/libsystemd-network/dhcp6-option.c:399
    #3 0x55f775788b96 in dhcp6_lease_set_domains src/libsystemd-network/sd-dhcp6-lease.c:225
    #4 0x55f775781348 in client_parse_message src/libsystemd-network/sd-dhcp6-client.c:807
    #5 0x55f775781ba2 in client_receive_advertise src/libsystemd-network/sd-dhcp6-client.c:895
    #6 0x55f775782453 in client_receive_message src/libsystemd-network/sd-dhcp6-client.c:994
    #7 0x7f13c7e447f4 in source_dispatch src/libsystemd/sd-event/sd-event.c:2268
    #8 0x7f13c7e471b0 in sd_event_dispatch src/libsystemd/sd-event/sd-event.c:2627
    #9 0x7f13c7e47ab3 in sd_event_run src/libsystemd/sd-event/sd-event.c:2686
    #10 0x7f13c7e47c21 in sd_event_loop src/libsystemd/sd-event/sd-event.c:2706
    #11 0x55f77577863c in test_client_solicit src/libsystemd-network/test-dhcp6-client.c:737
    #12 0x55f77577884b in main src/libsystemd-network/test-dhcp6-client.c:760
    #13 0x7f13c7589400 in __libc_start_main (/lib64/libc.so.6+0x20400)

SUMMARY: AddressSanitizer: 8 byte(s) leaked in 2 allocation(s).
```