fdo#71429: sw: fix crashes when changing header first sharing
fdo#71429: sw: fix crashes when changing header first sharing

Copy some nutso code in SwUndoPageDesc::ExchangeContentNodes() to
work on the un-shared First header/footer too, which apparently avoids
the crash.  It's not like Undo of header/footer isn't already a
house of cards anyway.

Change-Id: Ie6593c4784ce9d368a5098ffb3aa4dec536d250e

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core