Always allow timestamps to be printed

If the timestamp is above 9999-12-30, (or 2038-something-something on 32 bit),
use XXXX-XX-XX XX:XX:XX as the replacement.

The problem with refusing to print timestamps is that our code accepts such
timestamps, so we can't really just refuse to process them afterwards. Also, it
makes journal files non-portable, because suddently we might completely refuse
to print entries which are totally OK on a different machine.

src/basic/time-util.c

@@ -282,8 +282,11 @@ static char *format_timestamp_internal(
                 return NULL; /* Timestamp is unset */
 
         /* Let's not format times with years > 9999 */
-        if (t > USEC_TIMESTAMP_FORMATTABLE_MAX)
-                return NULL;
+        if (t > USEC_TIMESTAMP_FORMATTABLE_MAX) {
+                assert(l >= strlen("--- XXXX-XX-XX XX:XX:XX") + 1);
+                strcpy(buf, "--- XXXX-XX-XX XX:XX:XX");
+                return buf;
+        }
 
         sec = (time_t) (t / USEC_PER_SEC); /* Round down */