Checking that b:undo_ftplugin exists before appending to it

This plugin expects b:undo_ftplugin to exists which is not always the case. I made a very simple fix and it's very well possible that it can be written more succintly and idiomatically - but it worked for me :)
Merge pull request #119 from dkvasnicka/patch-1

Checking that b:undo_ftplugin exists before appending to it