tdf#109190 sd: cancel selecting status on new mousedown

It is possible to cancel a mouse-down without a mouse-up
(for example with an ESC button).
So, cancel an obsolete "MouseSelecting" on mouse-down.

Change-Id: I37d641612cf2cd8099af8f04b12fdb364c4f9e24
Reviewed-on: https://gerrit.libreoffice.org/56857
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>

sd/source/ui/view/drviews4.cxx

@@ -270,6 +270,8 @@ void DrawViewShell::MouseButtonDown(const MouseEvent& rMEvt,
     ::sd::Window* pWin)
 {
     mbMouseButtonDown = true;
+    mbMouseSelecting = false;
+
     // We have to check if a context menu is shown and we have an UI
     // active inplace client. In that case we have to ignore the mouse
     // button down event. Otherwise we would crash (context menu has been