journal-remote: bump the refcount right after creating the writer object

Otherwise we might hit an assertion during cleanup if the
following mmap_cache_new() call fails:

Assertion 'p->n_ref > 0' failed at src/journal-remote/journal-remote-write.c:80, function writer_unref(). Aborting.

==2069==ERROR: AddressSanitizer: ABRT on unknown address 0x000000000815 (pc 0x7f39dcd0200b bp 0x7ffe2fe24db0 sp 0x7ffe2fe24b60 T0)
SCARINESS: 10 (signal)
    #0 0x7f39dcd0200b in raise (/lib/x86_64-linux-gnu/libc.so.6+0x4300b) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)
    #1 0x7f39dcce1858 in abort (/lib/x86_64-linux-gnu/libc.so.6+0x22858) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)
    #2 0x7f39dd747e49 in log_assert_failed /work/build/../../src/systemd/src/basic/log.c:940:9
    #3 0x4e4431 in writer_unref /work/build/../../src/systemd/src/journal-remote/journal-remote-write.c:80:1
    #4 0x4e3fd5 in writer_unrefp /work/build/../../src/systemd/src/journal-remote/journal-remote-write.h:27:1
    #5 0x4e3fd5 in writer_new /work/build/../../src/systemd/src/journal-remote/journal-remote-write.c:56:1
    #6 0x4e04bc in journal_remote_get_writer /work/build/../../src/systemd/src/journal-remote/journal-remote.c:125:21
    #7 0x4e0e0b in get_source_for_fd /work/build/../../src/systemd/src/journal-remote/journal-remote.c:181:13
    #8 0x4e0e0b in journal_remote_add_source /work/build/../../src/systemd/src/journal-remote/journal-remote.c:233:13
    #9 0x4df99f in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/journal-remote/fuzz-journal-remote.c:54:9
    #10 0x4e8f48 in NaloFuzzerTestOneInput (/build/fuzz-journal-remote+0x4e8f48)
    #11 0x507f53 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #12 0x50773a in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:514:3
    #13 0x508e09 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:757:19
    #14 0x509ad5 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:895:5
    #15 0x4f8e3f in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    #16 0x4f9708 in LLVMFuzzerRunDriver /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:925:10
    #17 0x4e91c5 in main (/build/fuzz-journal-remote+0x4e91c5)
    #18 0x7f39dcce3082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)
    #19 0x420bcd in _start (/build/fuzz-journal-remote+0x420bcd)

DEDUP_TOKEN: raise--abort--log_assert_failed
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: ABRT (/lib/x86_64-linux-gnu/libc.so.6+0x4300b) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee) in raise

Found by Nallocufzz.

src/journal-remote/journal-remote-write.c

@@ -32,13 +32,13 @@ int writer_new(RemoteServer *server, Writer **ret) {
         if (!w)
                 return -ENOMEM;
 
+        w->n_ref = 1;
         w->metrics = server->metrics;
 
         w->mmap = mmap_cache_new();
         if (!w->mmap)
                 return -ENOMEM;
 
-        w->n_ref = 1;
         w->server = server;
 
         if (is_dir(server->output, /* follow = */ true) > 0) {