journald: Increase stdout buffer size sooner, when almost full

If the previous received buffer length is almost equal to the allocated
buffer size, before this change the next read can only receive a couple
of bytes (in the worst case only 1 byte), which is not efficient.

(cherry picked from commit 034e971)

Related: #2029426

src/journal/journald-stream.c

@@ -507,8 +507,8 @@ static int stdout_stream_process(sd_event_source *es, int fd, uint32_t revents,
                 goto terminate;
         }
 
-        /* If the buffer is full already (discounting the extra NUL we need), add room for another 1K */
-        if (s->length + 1 >= s->allocated) {
+        /* If the buffer is almost full, add room for another 1K */
+        if (s->length + 512 >= s->allocated) {
                 if (!GREEDY_REALLOC(s->buffer, s->allocated, s->length + 1 + 1024)) {
                         log_oom();
                         goto terminate;