Fix a crash when copying flex insets to another document.

Fix a crash when copying flex insets to another document.

I know that Inset has a buffer, but we should be careful when handling Insets in CutAndPaste, these don't always have a buffer. Besides, we are in the function switchBetweenClasses(oldone, newone). Then it's pretty strange to ask the flex inset whether it is defined in its buffer->textclass, while we have an explicit new textclass defined.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@31223 a592a061-630c-0410-9148-cb99ea01b6c8