Don't allow jobs with zero ttr.

doc/protocol.txt

@@ -129,6 +129,8 @@ It inserts a job into the queue.
    to run this job. This time is counted from the moment a worker reserves
    this job. If the worker does not delete, release, or bury the job within
    <ttr> seconds, the job will time out and the server will release the job.
+   The minimum ttr is 1. If the client sends 0, the server will silently
+   increase the ttr to 1.
 
  - <bytes> is an integer (currently must be < 2**16) indicating the size of
    the job body, not including the trailing "\r\n".

prot.c

@@ -797,7 +797,7 @@ dispatch_cmd(conn c)
 
         conn_set_producer(c);
 
-        c->in_job = make_job(pri, delay, ttr, body_size + 2);
+        c->in_job = make_job(pri, delay, ttr ? : 1, body_size + 2);
 
         fill_extra_data(c);