Fix source_from_range crash on invalid source range

There is a case where the Sass parser reports the end of a source range
on a non-existent line. One such example is the code below, which has no
trailing semicolon:

    @import 'file'

Add a check to the `source_from_range` method to gracefully handle this
edge case. I'm refraining from doing a blanket check for _any_ invalid
source range as I want to know the actual cases when this happens.

Change-Id: I300f3847bb77ed22eac21e89480d863aece2f75c
Reviewed-on: https://gerrit.causes.com/32914
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>