Construct grid fails if rectangle has any rounding (rx, ry)

Summary:

Problems to construct a grid when you try to create one it appeared a long line instead a grid

Steps to reproduce:

• open Inkscape
• draw a rectangle
• set rectangle's Rx or Ry to 0.001px
• open Path Effects
• add Construct Grid path effect

What happened?

A long line appeared instead of a grid, in my case I was going to make a grid sheet.

What should have happened?

I should have been able to create the grid without any problems.

Debug info

• 0.92.4 (5da689c313, 2019-01-14)