Fix a crash following the input of an invalid paragraph separation value in the docum...

Fix a crash following the input of an invalid paragraph separation value in the document settings dialog (bug 4556).

* src/frontends/qt4/GuiDocument.cpp:
	- disallow input of defskip length without actual length value.

* src/BufferParams.cpp (readToken):
	- there might be documents with the invalif "\defskip defskip" param. Reset this to "\defskip medskip".

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23022 a592a061-630c-0410-9148-cb99ea01b6c8