Fix bug 5785: http://bugzilla.lyx.org/show_bug.cgi?id=5785.

Fix bug 5785: http://bugzilla.lyx.org/show_bug.cgi?id=5785.
Reference dialog update crash for read-only documents.

Comments:
* -1 is a valid return of bufferCO->currentIndex() so we need to test for it.
* set the correct enabling of the button in case no call to updateContents follows.
* remove unneeded call to setFocusProxy. The widget is either disabled or the focus proxy is set in updateRefs.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@28547 a592a061-630c-0410-9148-cb99ea01b6c8