#1304 OSLIB Jobs Queues: return JOB_NULL descriptors to the free pool

[Giovanni Di Sirio]

The OSLIB jobs dispatcher was leaking job descriptors on the JOB_NULL control
path.

In both chJobDispatch() and chJobDispatchTimeout(), a fetched descriptor was
returned to the guarded pool only when jobfunc != NULL. When a null job was
used as the documented worker shutdown signal, the dispatcher returned
MSG_JOB_NULL but did not release the descriptor back to the free pool.

Impact:

• each null shutdown job permanently consumed one descriptor
• repeated worker shutdown cycles could exhaust the queue
• disposal-time checks did not detect the loss because the queue could still
  appear idle

The fix:

• always return the fetched descriptor to the guarded pool, including the
  MSG_JOB_NULL path
• add a regression test that posts null shutdown jobs, waits for the workers
  to exit, then verifies:
  • the guarded pool counter is back to JOBS_QUEUE_SIZE
  • the mailbox has no pending jobs