Ignore resetting to invalid locale (Closes: #377)

[jspricke]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
  - [ ] I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Resetting the locale was introduced in 5dcf236, before that an invalid
locale was ignored. Restore the old behavior insofar that only valid
locales are restored.
This fixes the unit tests of scottkosty/vit with unset LANG.

[coveralls]

Coverage decreased (-0.01%) to 77.852% when pulling fd010f8 on jspricke:try_setlocale into 56194f9 on urwid:master.

[jspricke]

An example of the broken test can be found here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=963315