Fix off-by-one bug in MarkerHandler

This issue has been pointed out in
https://github.com/eclipsesource/jshint-eclipse/pull/51#issuecomment-17765331
Protect against illegal line numbers

There have been reports [1,2] about IOOBEs because of non-existing line
numbers while processing the errors reported by JSHint.
I've never managed to reproduce these problems, however, the plug-in
should not crash in case of unexpected input.

Add a safeguard in JSHint.createProblem(), add unit tests.

[1] https://github.com/eclipsesource/jshint-eclipse/issues/27
[2] https://github.com/eclipsesource/jshint-eclipse/issues/60