test: reinitialize arg_transport before parsing arguments - #30806

Since libfuzzer feeds a single fuzzing process with multiple inputs, we
might carry over arg_transport from a previous invocation, tripping over
the assert in acquire_bus():

+----------------------------------------Release Build Stacktrace----------------------------------------+
Assertion 'transport != BUS_TRANSPORT_REMOTE || runtime_scope == RUNTIME_SCOPE_SYSTEM' failed at src/shared/bus-util.c:284, function bus_connect_transport(). Aborting.
AddressSanitizer:DEADLYSIGNAL
=================================================================
==2739==ERROR: AddressSanitizer: ABRT on unknown address 0x00000ab3 (pc 0xf7f52509 bp 0xffdf74cc sp 0xffdf74b0 T0)
SCARINESS: 10 (signal)
    #0 0xf7f52509 in linux-gate.so.1
    #1 0xf703b415 in raise
    #2 0xf70233f6 in abort
    #3 0xf772ac0a in log_assert_failed systemd/src/basic/log.c:968:9
    #4 0xf77300d5 in log_assert_failed_return systemd/src/basic/log.c:987:17
    #5 0xf7432bbf in bus_connect_transport systemd/src/shared/bus-util.c:284:9
    #6 0x818cd17 in acquire_bus systemd/src/systemctl/systemctl-util.c:53:29
    #7 0x815fd3c in help_boot_loader_entry systemd/src/systemctl/systemctl-logind.c:431:13
    #8 0x819ca87 in systemctl_parse_argv systemd/src/systemctl/systemctl.c:863:37
    #9 0x8197632 in systemctl_dispatch_parse_argv systemd/src/systemctl/systemctl.c:1137:16
    #10 0x813328d in LLVMFuzzerTestOneInput systemd/src/systemctl/fuzz-systemctl-parse-argv.c:54:13
    #11 0x81bbe7e in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned int) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #12 0x81bb5b8 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned int, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:514:3
    #13 0x81bd42d in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:826:7
    #14 0x81bd62e in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:857:3
    #15 0x81ac84c in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned int)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    #16 0x81d65c7 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #17 0xf7024ed4 in __libc_start_main
    #18 0x806bdb5 in _start

Resolves: systemd#30802