Fix crash in LeadingZero

Due to an upstream Sass bug (https://github.com/sass/sass/issues/1343),
check for the case where a source range is incorrect and pass if it is.

This means we may not report some lints, but this case should be
relatively rare it's better than crashing.

Change-Id: I079e6a6681400ed574a9dda035c9555748188ee1
Reviewed-on: http://gerrit.causes.com/41017
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>