Avoid endless loop when the window is too narrow

Avoid endless loop when the window is too narrow

 * breakRow: remove wrong condition that would silently eat the contents of the
   paragraph when the window is narrower than left margin

 * breakRow: make sure that there is at least one element in each row

 * breakAt: when force-breaking a row element, make sure it is not empty. Doing
   so may create empty rows and therefore a endless loop.

Fixes bugs #9962 and #10001.