fix endless loop in KLineEdit->setSqueezedText()

If KLineEdit has squeezed text enabled, but the widget has still zero width, KLineEdit::setSqueezedText() can go into an endless loop trying to remove more than all letters from the string. This patch fixes both this underrun and the corresponding possible overrun if the squeezed text is too small. The underrun is responsible for endless loops in digikam, see the corresponding bug report.

REVIEW: 114519
BUG: 328263