def: 6112 only switch to preview mode after buffer is empty

git-svn-id: svn+ssh://kelev.kaltura.com/usr/local/kalsource/front/workspaces/krecord/trunk@93152 6b8eccd3-e8c5-4e7d-8186-e12b5326b719
fwr: v5.1.13

git-svn-id: svn+ssh://kelev.kaltura.com/usr/local/kalsource/front/workspaces/krecord/trunk@93153 6b8eccd3-e8c5-4e7d-8186-e12b5326b719