system-config-users reports UnicodeDecodeError: 'utf8' codec can't decode bytes in position X-Y: invalid data

Issue

• system-config-users crashes with a UnicodeDecodeError if there are non-Unicode symbols in /etc/passwd (or the related files /etc/group, /etc/shadow, or /etc/gshadow)

# /usr/bin/system-config-users
Traceback (most recent call last):
  File "/usr/share/system-config-users/system-config-users.py", line 45, in ?
    mainWindow.mainWindow()
  File "/usr/share/system-config-users/mainWindow.py", line 266, in __init__
    self.refresh()
  File "/usr/share/system-config-users/mainWindow.py", line 427, in refresh
    self.populate_lists()
  File "/usr/share/system-config-users/mainWindow.py", line 518, in populate_lists
    self.populate_user_list()
  File "/usr/share/system-config-users/mainWindow.py", line 458, in populate_user_list
    gecos = unicode (gecos, 'utf-8')
UnicodeDecodeError: 'utf8' codec can't decode bytes in position 11-13: invalid data