Handle special values for offset in 'Qt5AccessibleWidget::attributes'
the same way that the base class's 'QAccessibleTextWidget::attributes'
does, s. [1].

In particular, an offset matching the text length can
be passed e.g. by "accerciser" or screen readers,
which previously resulted in an 'IndexOutOfBoundsException'
being thrown when the index was later checked in the
call to 'VCLXAccessibleStatusBarItem::getCharacterAttributes'.

See also 'IAccessibleText::attributes' documentation at [2] and
the page on special offset values referenced there [3].

[1] https://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/accessible/qaccessiblewidgets.cpp?h=5.12.4#n791
[2] https://accessibility.linuxfoundation.org/a11yspecs/ia2/docs/html/interface_i_accessible_text.html#a29e5c8f69ec13c683ed6bca53333e6a5
[3] https://accessibility.linuxfoundation.org/a11yspecs/ia2/docs/html/_general_info.html#_specialOffsets

Change-Id: I623995aeb689b31c5b49fb3ace8e4dd4c18927d2
Reviewed-on: https://gerrit.libreoffice.org/73225
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>