dhcp6: fix an off-by-one error in dhcp6_option_parse_domainname

dhcp6: fix an off-by-one error in dhcp6_option_parse_domainname

==14==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200055fa9c at pc 0x0000005458f1 bp 0x7ffc78940d90 sp 0x7ffc78940d88
READ of size 1 at 0x60200055fa9c thread T0
    #0 0x5458f0 in dhcp6_option_parse_domainname /work/build/../../src/systemd/src/libsystemd-network/dhcp6-option.c:555:29
    #1 0x54706e in dhcp6_lease_set_domains /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp6-lease.c:242:13
    #2 0x53fce0 in client_parse_message /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp6-client.c:984:29
    #3 0x53f3bc in client_receive_advertise /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp6-client.c:1083:13
    #4 0x53d57f in client_receive_message /work/build/../../src/systemd/src/libsystemd-network/sd-dhcp6-client.c:1182:21
    #5 0x7f0f7159deee in source_dispatch /work/build/../../src/systemd/src/libsystemd/sd-event/sd-event.c:3042:21
    #6 0x7f0f7159d431 in sd_event_dispatch /work/build/../../src/systemd/src/libsystemd/sd-event/sd-event.c:3455:21
    #7 0x7f0f7159ea8d in sd_event_run /work/build/../../src/systemd/src/libsystemd/sd-event/sd-event.c:3512:21
    #8 0x531f2b in fuzz_client /work/build/../../src/systemd/src/fuzz/fuzz-dhcp6-client.c:44:9
    #9 0x531bc1 in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/fuzz/fuzz-dhcp6-client.c:53:9
    #10 0x57bec8 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:570:15
    #11 0x579d67 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) /src/libfuzzer/FuzzerLoop.cpp:479:3
    #12 0x57dc92 in fuzzer::Fuzzer::MutateAndTestOne() /src/libfuzzer/FuzzerLoop.cpp:707:19
    #13 0x580ca6 in fuzzer::Fuzzer::Loop(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, fuzzer::fuzzer_allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) /src/libfuzzer/FuzzerLoop.cpp:838:5
    #14 0x55e968 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:764:6
    #15 0x551a1c in main /src/libfuzzer/FuzzerMain.cpp:20:10
    #16 0x7f0f701a082f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #17 0x41e928 in _start (/out/fuzz-dhcp6-client+0x41e928)