coredump: fix message when we fail to save a journald coredump

If creation of the message failed, we'd write a bogus entry:
systemd-coredump[1400]: Cannot store coredump of 416 (systemd-journal): No space left on device
systemd-coredump[1400]: MESSAGE=Process 416 (systemd-journal) of user 0 dumped core.
systemd-coredump[1400]: Coredump diverted to

src/coredump/coredump.c

@@ -794,8 +794,8 @@ log:
         core_message = strjoin("MESSAGE=Process ", context[CONTEXT_PID],
                                " (", context[CONTEXT_COMM], ") of user ",
                                context[CONTEXT_UID], " dumped core.",
-                               journald_crash ? "\nCoredump diverted to " : NULL,
-                               journald_crash ? filename : NULL);
+                               journald_crash && filename ? "\nCoredump diverted to " : NULL,
+                               journald_crash && filename ? filename : NULL);
         if (!core_message)
                 return log_oom();