[gpad] always set fXUpNDC/fYUpNDC members in TPad::SetPad() (#4089)
[gpad] always set fXUpNDC/fYUpNDC members in TPad::SetPad() (#4089)

* [gpad] set X/Y UpNDC in TPad::SetPad

Otherwise direct call to SetBoxX1/SetBoxY1 will fail.
Only works now while fXUpNDC recalculated when start interactive
movement of frame

* [gpad] use nullptr to initialize all pointer-related members

olifre/root,karies/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,root-mirror/root,karies/root