sd-network: fix memleak in dhcp6_lease_set_domains (#5113)

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..4e14d8f 100644
--- a/src/libsystemd-network/test-dhcp6-client.c
+++ b/src/libsystemd-network/test-dhcp6-client.c
@@ -286,6 +286,8 @@ static int test_advertise_option(sd_event *e) {
                         assert_se(optlen == 11);
                         assert_se(dhcp6_lease_set_domains(lease, optval,
                                                           optlen) >= 0);
+                        assert_se(dhcp6_lease_set_domains(lease, optval,
+                                                          optlen) >= 0);
                         break;

                 case SD_DHCP6_OPTION_SNTP_SERVERS:
```

Fixes:
```
==27369==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 10 byte(s) in 1 object(s) allocated from:
    #0 0x7f90e7d21160 in strdup (/lib64/libasan.so.3+0x5a160)
    #1 0x7f90e7467f69 in strv_extend src/basic/strv.c:552
    #2 0x5612fcc19379 in dhcp6_option_parse_domainname src/libsystemd-network/dhcp6-option.c:399
    #3 0x5612fcc1acdf in dhcp6_lease_set_domains src/libsystemd-network/sd-dhcp6-lease.c:225
    #4 0x5612fcc06b95 in test_advertise_option src/libsystemd-network/test-dhcp6-client.c:287
    #5 0x5612fcc0a987 in main src/libsystemd-network/test-dhcp6-client.c:761
    #6 0x7f90e6d46400 in __libc_start_main (/lib64/libc.so.6+0x20400)

SUMMARY: AddressSanitizer: 10 byte(s) leaked in 1 allocation(s).
```

(cherry picked from commit 0b75a95a)