Fix crash in inline reflower when child reflow moves all children
Fix crash in inline reflower when child reflow moves all children

The fix in #2822 catered for cases where the frame itself is moved and
reset. When a page break occurs before the first child, which can
happen when an inline frame contains a block frame, the frame is left
empty after child reflow. Make sure to not crash in that case.

The frame is left without position, but as empty inline frames are not
drawn currently, this should be fine for now.

Fixes #2763

dompdf/dompdf,dompdf/dompdf