Editor: Do not highlight invalid Uses

Invalid Uses (line == 0) are generated by e.g. a Q_D statement.

This kills
 1. the selection artifact in line 0, column 0 and
 2. the warning "QTextCursor::setPosition: Position '-1' out of range"

for the following example code and cursor position:

    void f()
    {
        Q_D(x)
        d->hello(); // put the cursor on the 'd'!
    }

Change-Id: I6968cbc261f05e08f71ff125437c8c9e863b475b
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>