journald: add assert for allocated buffer size

Coverity flags allocated - 1 as a potential underflow when
allocated is 0. After GREEDY_REALLOC succeeds the buffer is
guaranteed non-empty, but Coverity cannot trace through the
conditional. Add an assert to document this.

CID#1548053

Follow-up for ec20fe5ffb

src/journal/journald-stream.c

@@ -557,6 +557,8 @@ static int stdout_stream_process(sd_event_source *es, int fd, uint32_t revents,
 
         /* Try to make use of the allocated buffer in full, but never read more than the configured line size. Also,
          * always leave room for a terminating NUL we might need to add. */
+        /* Silence static analyzers, GREEDY_REALLOC above ensures allocated > 0 */
+        assert(allocated > 0);
         limit = MIN(allocated - 1, MAX(s->manager->config.line_max, STDOUT_STREAM_SETUP_PROTOCOL_LINE_MAX));
         assert(s->length <= limit);
         iovec = IOVEC_MAKE(s->buffer + s->length, limit - s->length);