Fix ProblemHighlighter::setProblems() method

The patch provides:

1. Assertion fix, which happens when lineString is empty or contains only space-characters (loop at line 162). Such lines detected for example by vera++ analyzer (warning "L005: too many consecutive empty lines").

2. Highlighting fix - old version doesn't highlight last line character.

(cherry picked from commit 27967a570c230a01defb1cb960fce1c626c7c837)