check command state before accessing return value

[vladak]

When mirroring a JDK project with unresponsive Mercurial parent, the opengrok-mirror command failed due to timeout which caused TypeError exception due to the status value being None:

processing of project 'jdk8' failed
  failed commands:
    'sudo -u wsmirror /opengrok/dist/bin/venv/bin/opengrok-mirror --api_timeout 16 --backupcount 120 --workers 3 --strip-outgoing -I -b -c /opengrok/etc/mirror-config.yml jdk8': 1
      multiprocessing.pool.RemoteTraceback:      """      Traceback (most recent call last):        
File "/usr/lib/python3.9/multiprocessing/pool.py", line 125, in worker        
  result = (True, func(*args, **kwds))        
File "/usr/lib/python3.9/multiprocessing/pool.py", line 48, in mapstar        
  return list(map(*args))        
File "/opengrok/dist/bin/venv/lib/python3.9/site-packages/opengrok_tools/mirror.py", line 77, in worker        
  return mirror_project(config, project_name,        
File "/opengrok/dist/bin/venv/lib/python3.9/site-packages/opengrok_tools/utils/mirror.py", line 545, in mirror_project        
  r = process_outgoing(repos, project_name)        
File "/opengrok/dist/bin/venv/lib/python3.9/site-packages/opengrok_tools/utils/mirror.py", line 431, in process_outgoing        
  if repo.strip_outgoing():        
File "/opengrok/dist/bin/venv/lib/python3.9/site-packages/opengrok_tools/scm/mercurial.py", line 158, in strip_outgoing        
  if status > 0:      
TypeError: '>' not supported between instances of 'NoneType' and 'int'      """        
    The above exception was the direct cause of the following exception:        
    Traceback (most recent call last):        
File "/opengrok/dist/bin/venv/bin/opengrok-mirror", line 8, in <module>        
  sys.exit(main())        
File "/opengrok/dist/bin/venv/lib/python3.9/site-packages/opengrok_tools/mirror.py", line 227, in main        
  project_results = pool.map(worker, worker_args, 1)        
File "/usr/lib/python3.9/multiprocessing/pool.py", line 364, in map        
  return self._map_async(func, iterable, mapstar, chunksize).get()        
File "/usr/lib/python3.9/multiprocessing/pool.py", line 771, in get        
  raise self._value      TypeError: '>' not supported between instances of 'NoneType' and 'int'

This change fixes that.

[kulikjak]

LGTM