ValueError: invalid literal for int() with base 10: '$24207'
#### How to Reproduce
While doing a POST operation on `/ticket/24040`, Trac issued an internal error.
_(please provide additional details here)_
Request parameters:
```
{u'__FORM_TOKEN': u'6ed886568305dff9d4e16c75',
u'action': u'leave',
u'comment': u'',
u'field_actualpoints': u'',
u'field_component': u'Applications/Tor Browser',
u'field_keywords': u'tbb-crash, TorBrowserTeam201712',
u'field_milestone': u'',
u'field_parent': u'$24207',
u'field_points': u'',
u'field_priority': u'High',
u'field_reviewer': u'',
u'field_severity': u'Major',
u'field_sponsor': u'',
u'field_summary': u'TorBrowser crashes at riot.im/app',
u'field_type': u'defect',
u'field_version': u'',
'id': u'24040',
u'preview': u'Preview',
u'replyto': u'',
u'sfp_email': u'',
u'sfph_mail': u'',
u'start_time': u'1512985891144518',
u'view_time': u'1512985891144518'}
```
User agent: `Mozilla/5.0 (Windows NT 6.1; rv:52.0) Gecko/20100101 Firefox/52.0`
#### System Information
_System information not available_
#### Enabled Plugins
_Plugin information not available_
#### Interface Customization
_Interface customization information not available_
#### Python Traceback
```
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/trac/web/main.py", line 613, in _dispatch_request
dispatcher.dispatch(req)
File "/usr/lib/python2.7/dist-packages/trac/web/main.py", line 253, in dispatch
resp = chosen_handler.process_request(req)
File "/usr/lib/python2.7/dist-packages/trac/ticket/web_ui.py", line 177, in process_request
return self._process_ticket_request(req)
File "/usr/lib/python2.7/dist-packages/trac/ticket/web_ui.py", line 661, in _process_ticket_request
valid = self._validate_ticket(req, ticket, not valid) and valid
File "/usr/lib/python2.7/dist-packages/trac/ticket/web_ui.py", line 1378, in _validate_ticket
for field, message in manipulator.validate_ticket(req, ticket):
File "build/bdist.linux-x86_64/egg/childtickets/childtickets.py", line 69, in validate_ticket
pid = int(ticket.values.get('parent').lstrip('#'))
ValueError: invalid literal for int() with base 10: '$24207'
```
issue