gerber export emits an error with covered pads

If a pad is completely covered with solder mask. The resulting gerbers look fine when viewd with gerbv.

To reproduce:

1) open pcb
2) draw a track
3) copy track to buffer
4) do convert-buffer-to-element
5) paste buffer contents to canvas
6) export gerbers
An error message appears on stdout:
      error: aperture for width 0 type ROUND is 0

Suggestion: Accept zero size solder mask gracefully.

---<)kaimartin(>---