in some complex cases, results of drag_value_get could be invalid.
because variables to calculate the values could change.

Assume that below functionalities are executed in sequence.

content_region_show -> content_resized -> bar_read_and_update

because a formula to calculate vx is that vx = px / (minx + maxx),
and maxx = content width - pan width, vx can not be used to adjust pan position.

this patch check variables to verify vx/y values