Fix crash reported by Tommaso:

Fix crash reported by Tommaso:
  http://marc.info/?l=lyx-devel&m=133876924408431&w=2
The problem here is that the copy_params() routine in FindAndReplace.cpp
created a new DocumentClass, but it never updated its Buffer to reflect
that new DocumentClass. So its Paragraphs still contained points to the
Layouts in the old DocumentClass which, since ead697d4b6c7, gets garbage
collected once it is no longer needed. So the Layout doesn't exist, and
we crash.