Fix error after resizing the document if there was an action preview

If you confirmed the dialog with Enter after hovering over the canvas,
previously it got:

    IndexError: list index out of range

when canceling the preview after it became outdated in regard to the
canvas size.

src/textual_paint/paint.py

@@ -2316,6 +2316,8 @@ class PaintApp(App[None]):
                     if width < 1 or height < 1:
                         raise ValueError
 
+                    self.cancel_preview()
+
                     # TODO: DRY undo state creation (at least the undos/redos part)
                     action = Action(_("Attributes"), Region(0, 0, self.image.width, self.image.height))
                     action.is_resize = True