Fixing crash condition in ExtDateWidget. If initialized without a date

Fixing crash condition in ExtDateWidget.  If initialized without a date
argument, it was initialized to an invalid date, which caused a crash in the
function ExtDate::daysInMonth().  Two fixes make the widget much more robust:

+ ExtDate::daysInMonth() and ExtDate::leapYear() now check the validity of the
date first, and return sensible default values if the date is invalid.

+ the default ctor ExtDateWidget() now initializes to the current date, rather
than an invalid date.

This fixes the crash in the ScriptBuilder tool.

CCMAIL: kstars-devel@kde.org

svn path=/trunk/kdeedu/libkdeedu/; revision=316636