QgsRendererPropertiesDialog: initialize mDockMode member

Otherwise when adding a layer for the first time, Valgrind raises
the following warning:

==27708== Conditional jump or move depends on uninitialised value(s)
==27708==    at 0x689F04D: QgsRendererWidget::setDockMode(bool) (qgsrendererwidget.cpp:354)
==27708==    by 0x68B7456: QgsSingleSymbolRendererWidget::setDockMode(bool) (qgssinglesymbolrendererwidget.cpp:110)
==27708==    by 0x68972F9: QgsRendererPropertiesDialog::rendererChanged() (qgsrendererpropertiesdialog.cpp:273)
==27708==    by 0x689C7A6: QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, void (QgsRendererPropertiesDialog::*)()>::call(void (QgsRendererPropertiesDialog::*)(), QgsRendererPropertiesDialog*, void**) (qobjectdefs_impl.h:136)
==27708==    by 0x689C675: void QtPrivate::FunctionPointer<void (QgsRendererPropertiesDialog::*)()>::call<QtPrivate::List<>, void>(void (QgsRendererPropertiesDialog::*)(), QgsRendererPropertiesDialog*, void**) (qobjectdefs_impl.h:169)
==27708==    by 0x689C1D0: QtPrivate::QSlotObject<void (QgsRendererPropertiesDialog::*)(), QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobject_impl.h:120)
==27708==    by 0xB17F8B5: QMetaObject::activate(QObject*, int, int, void**) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xA569F10: QComboBox::currentIndexChanged(int) (in /opt/qt59/lib/libQt5Widgets.so.5.9.1)
==27708==    by 0xA56C280: ??? (in /opt/qt59/lib/libQt5Widgets.so.5.9.1)
==27708==    by 0xA56EA6C: ??? (in /opt/qt59/lib/libQt5Widgets.so.5.9.1)
==27708==    by 0xA56EC8E: QComboBox::setCurrentIndex(int) (in /opt/qt59/lib/libQt5Widgets.so.5.9.1)
==27708==    by 0x6897DA7: QgsRendererPropertiesDialog::syncToLayer() (qgsrendererpropertiesdialog.cpp:383)
==27708==    by 0x68961EF: QgsRendererPropertiesDialog::QgsRendererPropertiesDialog(QgsVectorLayer*, QgsStyle*, bool, QWidget*) (qgsrendererpropertiesdialog.cpp:120)
==27708==    by 0x54BEAB8: QgsLayerStylingWidget::updateCurrentWidgetLayer() (qgslayerstylingwidget.cpp:480)
==27708==    by 0x54C51DB: QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, void (QgsLayerStylingWidget::*)()>::call(void (QgsLayerStylingWidget::*)(), QgsLayerStylingWidget*, void**) (qobjectdefs_impl.h:136)
==27708==    by 0x54C4ED7: void QtPrivate::FunctionPointer<void (QgsLayerStylingWidget::*)()>::call<QtPrivate::List<>, void>(void (QgsLayerStylingWidget::*)(), QgsLayerStylingWidget*, void**) (qobjectdefs_impl.h:169)
==27708==    by 0x54C49DA: QtPrivate::QSlotObject<void (QgsLayerStylingWidget::*)(), QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobject_impl.h:120)
==27708==    by 0xB17F8B5: QMetaObject::activate(QObject*, int, int, void**) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xA717170: QListWidget::currentRowChanged(int) (in /opt/qt59/lib/libQt5Widgets.so.5.9.1)
==27708==    by 0xA717ABE: ??? (in /opt/qt59/lib/libQt5Widgets.so.5.9.1)
==27708==    by 0xB17F588: QMetaObject::activate(QObject*, int, int, void**) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xB102CD9: QItemSelectionModel::currentChanged(QModelIndex const&, QModelIndex const&) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xB102F5F: QItemSelectionModel::setCurrentIndex(QModelIndex const&, QFlags<QItemSelectionModel::SelectionFlag>) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xA717EFD: QListWidget::setCurrentRow(int) (in /opt/qt59/lib/libQt5Widgets.so.5.9.1)
==27708==    by 0x54BD527: QgsLayerStylingWidget::setLayer(QgsMapLayer*) (qgslayerstylingwidget.cpp:302)
==27708==    by 0x54C5134: QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<QgsMapLayer*>, void, void (QgsLayerStylingWidget::*)(QgsMapLayer*)>::call(void (QgsLayerStylingWidget::*)(QgsMapLayer*), QgsLayerStylingWidget*, void**) (qobjectdefs_impl.h:136)
==27708==    by 0x54C4E92: void QtPrivate::FunctionPointer<void (QgsLayerStylingWidget::*)(QgsMapLayer*)>::call<QtPrivate::List<QgsMapLayer*>, void>(void (QgsLayerStylingWidget::*)(QgsMapLayer*), QgsLayerStylingWidget*, void**) (qobjectdefs_impl.h:169)
==27708==    by 0x54C48FE: QtPrivate::QSlotObject<void (QgsLayerStylingWidget::*)(QgsMapLayer*), QtPrivate::List<QgsMapLayer*>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobject_impl.h:120)
==27708==    by 0xB17F8B5: QMetaObject::activate(QObject*, int, int, void**) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0x665A0B9: QgsMapLayerComboBox::layerChanged(QgsMapLayer*) (moc_qgsmaplayercombobox.cpp:209)
==27708==    by 0x7109BB7: QgsMapLayerComboBox::rowsChanged() (qgsmaplayercombobox.cpp:143)
==27708==    by 0x710A6B8: QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, void (QgsMapLayerComboBox::*)()>::call(void (QgsMapLayerComboBox::*)(), QgsMapLayerComboBox*, void**) (qobjectdefs_impl.h:136)
==27708==    by 0x710A585: void QtPrivate::FunctionPointer<void (QgsMapLayerComboBox::*)()>::call<QtPrivate::List<>, void>(void (QgsMapLayerComboBox::*)(), QgsMapLayerComboBox*, void**) (qobjectdefs_impl.h:169)
==27708==    by 0x710A4AA: QtPrivate::QSlotObject<void (QgsMapLayerComboBox::*)(), QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (qobject_impl.h:120)
==27708==    by 0xB17F8B5: QMetaObject::activate(QObject*, int, int, void**) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xB0F26AD: QAbstractItemModel::rowsInserted(QModelIndex const&, int, int, QAbstractItemModel::QPrivateSignal) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xB0FA71A: QAbstractItemModel::endInsertRows() (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xB11AF9A: ??? (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xB11D693: ??? (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==27708==    by 0xB121098: ??? (in /opt/qt59/lib/libQt5Core.so.5.9.1)

src/gui/symbology/qgsrendererpropertiesdialog.h

@@ -161,7 +161,7 @@ class GUI_EXPORT QgsRendererPropertiesDialog : public QDialog, private Ui::QgsRe
     QgsFeatureRequest::OrderBy mOrderBy;
 
   private:
-    bool mDockMode;
+    bool mDockMode = false;
 
     friend class QgsAppScreenShots;
 };