Fix "Allow null" in range widget (fixes #20831)

By default a range widget is built with a minimum value set to the
minimal integer that is possible to represent. When "allow null" is
enabled, a new value (minvalue - 1) is inserted. With the default
value, we then had an integer overflow.

(cherry picked from commit eb5a336)

Author: Hugo Mercier

src/gui/editorwidgets/qgsrangewidgetwrapper.cpp

@@ -145,7 +145,12 @@ void QgsRangeWidgetWrapper::initWidget( QWidget *editor )
     if ( allowNull )
     {
       int stepval = step.isValid() ? step.toInt() : 1;
-      minval -= stepval;
+      int newMinval = minval - stepval;
+      // make sure there is room for a new value (i.e. signed integer does not overflow)
+      if ( newMinval < minval )
+      {
+        minval = newMinval;
+      }
       mIntSpinBox->setValue( minval );
       QgsSpinBox *intSpinBox( qobject_cast<QgsSpinBox *>( mIntSpinBox ) );
       if ( intSpinBox )