tests: make fuzz-journal-remote less flaky

by always calling journal_remote_server_destroy, which resets global
variables like journal_remote_server_global. It should prevent crashes like
```
Assertion 'journal_remote_server_global == NULL' failed at src/journal-remote/journal-remote.c:312, function int journal_remote_server_init(RemoteServer *, const char *, JournalWriteSplitMode, _Bool, _Bool)(). Aborting.
AddressSanitizer:DEADLYSIGNAL
=================================================================
==24769==ERROR: AddressSanitizer: ABRT on unknown address 0x0539000060c1 (pc 0x7f23b4d5818b bp 0x7ffcbc4080c0 sp 0x7ffcbc407e70 T0)
SCARINESS: 10 (signal)
    #0 0x7f23b4d5818b in raise /build/glibc-eX1tMB/glibc-2.31/sysdeps/unix/sysv/linux/raise.c:51:1
    #1 0x7f23b4d37858 in abort /build/glibc-eX1tMB/glibc-2.31/stdlib/abort.c:79:7
    #2 0x7f23b5731809 in log_assert_failed systemd/src/basic/log.c:866:9
```

src/journal-remote/fuzz-journal-remote.c

@@ -21,7 +21,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_(unlink_tempfilep) char name[] = "/tmp/fuzz-journal-remote.XXXXXX.journal";
         _cleanup_close_ int fdout = -1;
         _cleanup_(sd_journal_closep) sd_journal *j = NULL;
-        RemoteServer s = {};
+        _cleanup_(journal_remote_server_destroy) RemoteServer s = {};
         int r;
 
         if (size <= 2)
@@ -59,7 +59,6 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         while (s.active)
                 assert_se(journal_remote_handle_raw_source(NULL, fdin, 0, &s) >= 0);
 
-        journal_remote_server_destroy(&s);
         assert_se(close(fdin) < 0 && errno == EBADF); /* Check that the fd is closed already */
 
         /* Out */