Fix for GDI crash if stroke-dasharray contains value<=0

Minor issue:

GDI Pen was crashing with some bad (Adobe Illustrator) data that had: `stroke-dasharray="10,10,0"`

Here is test case:

```
<?xml version="1.0" encoding="utf-8"?>
<!-- Generator: Adobe Illustrator 16.0.0, SVG Export Plug-In . SVG Version: 
6.00 Build 0)  -->
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg version="1.1" id="Version_1.0" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="400px" height="400px" viewBox="0 0 400 400" enable-background="new 0 0 400 400" xml:space="preserve">
<g id="main">
	<rect x="8.25" y="54.25" fill="none" stroke="#000000" stroke-width="5" stroke-dasharray="10,10,0" width="282" height="282"/>
	<circle fill="none" stroke="#000000" stroke-width="5" cx="149.25"  cy="195.25" r="79.5"/>
</g>
</svg>
```