Crash when the SVG width/height is set to auto #772

New Issue Checklist

• I have read and understood the CONTRIBUTING guide
• I have reproduced the problem while running in Xcode, and read the Console log, and acted upon any relevant error or warning messages
• I have reproduced the problem while running in Xcode's Debugger, and used breakpoints to examine the actual values of variables vs the expected values
• I am not using CocoaPods, or: I am using CocoaPods, but have verified my podspec points to SVGKit v2.x

Issue Description and Steps

In some cases when the SVG's width/height is set to auto the app crashes: example svg

*** Terminating app due to uncaught exception 'CALayerInvalidGeometry', reason: 'CALayer position contains NaN: [nan nan]'
I've found that if in CSSPrimitiveValue.m in the case of the unhandled CSS_STRING I return a non-zero value (f.e. 600) the app works correctly and SVG is rendered correctly.