test: initialize syslog_fd in fuzz-journald-kmsg too

This is a follow-up to 8857fb9b that prevents the fuzzer from crashing with
```
==220==ERROR: AddressSanitizer: ABRT on unknown address 0x0000000000dc (pc 0x7ff4953c8428 bp 0x7ffcf66ec290 sp 0x7ffcf66ec128 T0)
SCARINESS: 10 (signal)
    #0 0x7ff4953c8427 in gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x35427)
    #1 0x7ff4953ca029 in abort (/lib/x86_64-linux-gnu/libc.so.6+0x37029)
    #2 0x7ff49666503a in log_assert_failed_realm /work/build/../../src/systemd/src/basic/log.c:805:9
    #3 0x7ff496614ecf in safe_close /work/build/../../src/systemd/src/basic/fd-util.c:66:17
    #4 0x548806 in server_done /work/build/../../src/systemd/src/journal/journald-server.c:2064:9
    #5 0x5349fa in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/fuzz/fuzz-journald-kmsg.c:26:9
    #6 0x592755 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:571:15
    #7 0x590627 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) /src/libfuzzer/FuzzerLoop.cpp:480:3
    #8 0x594432 in fuzzer::Fuzzer::MutateAndTestOne() /src/libfuzzer/FuzzerLoop.cpp:708:19
    #9 0x5973c6 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:839:5
    #10 0x574541 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:764:6
    #11 0x5675fc in main /src/libfuzzer/FuzzerMain.cpp:20:10
    #12 0x7ff4953b382f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #13 0x420f58 in _start (/out/fuzz-journald-kmsg+0x420f58)
```