readLine() fails on long text lines (20000 chars)

Commit 1e6b424b692b20dcfec920f8d3563e520ec1ff05 introduced a regression when loading text files with long lines. I am using qt-4.6
branch.

The attached test case fails with long lines (LENGTH parameter 20000), but succeeds with short lines (e.g. only 16000 characters)