Were two issues:

• Divider was calculated in integer domain, causing rounding issues in general case, and causing singularity in a corner case when input is smaller than the preview size.

• The resolution was scaled down by 1 pixel for no obvious reason.

Compositor: Fix node preview when input resolution is not known

The final render will use scene resolution in this case.

For example, when Color Input is plugger to preview and composite output
nodes, final render will flood-fill the final image which is a size of
scene resolution with this color. Before this fix the node preview was
empty. After this fix the node preview will be flood-filled with the
color.