Avoid a possible crash when columns index is invalid

[zas]

• index < 0 was handled
• index >= len(self._columns) wasn't (it happened when switching between different branches, it shouldn't during normal use)
• so be more defensive and catch IndexError (and log it as warning)

Original error was:

Traceback (most recent call last):
  File "/home/zas/src/picard/picard/ui/widgets/configurablecolumnsheader.py", line 130, in on_sort_indicator_changed
    if index < 0 or not self._columns[index].sortable:
                        ~~~~~~~~~~~~~^^^^^^^
  File "/home/zas/src/picard/picard/ui/columns.py", line 167, in __getitem__
    return self._list.__getitem__(index)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IndexError: list index out of range

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other
• Describe this change in 1-2 sentences:

Problem

• JIRA ticket (optional): PICARD-XXX

Solution

Action

Additional actions required:

• Update Picard documentation (please include a reference to this PR)
• Other (please specify below)

[rdswift]

Good catch!

[phw]

LGTM