BUG Fixed divide by zero with SetRatioSize on missing image file
Fixes issue #2047
Merge pull request #2256 from tractorcow/3.1-setratiosize-dbz-fix

BUG Fixed divide by zero with SetRatioSize on missing image file
Merge branch '3.1' of git://github.com/silverstripe/silverstripe-framework into 3.1