patch 9.2.0594: Use-after-free with ":wqall" and a running terminal job

patch 9.2.0594: Use-after-free with ":wqall" and a running terminal job

Problem:  Using ":wqall" with a running terminal buffer can free the
          buffer that is currently being iterated over in the buffer
          list, resulting in a use-after-free (after v9.2.0593).
Solution: After stopping the job, check whether the buffer is still valid
          and restart the iteration from the first buffer if it was freed
          (Hirohito Higashi).

related: #20417
closes:  #20423

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>