Fix TypeError in indentation rule

[bgriffith]

Improved the code of the indentation rule with extra checks to stop a TypeError.

While this doesn't fix the reason (Gonzales-pe's handling of CRLF), it will stop the error breaking out and allow the linter to continue.

Reported in #389, #382, #373... and lots of other reports by Windows users.

/Users/ben/workspace/github/sass-lint/lib/rules/indentation.js:57
                  'line': node.content[i + 1].start.line,

TypeError: Cannot read property 'start' of undefined

DCO 1.1 Signed-off-by: Ben Griffith gt11687@gmail.com

[DanPurdy]

👍