Fix undistributed proxy not building if using custom directory.
I think the problem only happens when the custom directory is the clip directory.
Just tested and Sequences don’t work when custom is clip directory, but with clip editor proxy are build to the custom directory instead of custom directory + clipfilename folder.
Then it is the same issue as D6148 has fixed in VSE. I have tested functionality in clip editor, but haven't checked undistorted proxies...
This patch changes paths to proxies, this should be done only if absolutely necessary, because it will break existing projects.
I would suggest just printing error message.