Preferences child panels use wrong provider

[Peter Suter]

I think preference page child panels (#11853) are broken. (Possibly since [14097]?)

diff -r f44f904f5245 trac/prefs/web_ui.py
--- a/trac/prefs/web_ui.py	Sat Jun 09 00:11:51 2018 +0000
+++ b/trac/prefs/web_ui.py	Mon Jun 11 19:57:55 2018 +0200
@@ -96,6 +96,7 @@
         children = []
         if child_panels.get(panel_id):
             for name, label in child_panels[panel_id]:
+                provider = providers[name]
                 resp = provider.render_preference_panel(req, name)
                 ctemplate, cdata = resp[:2]
                 cdata.update(session_data)

Without this some random provider from a previous loop is used.