Red files in your library aren't corrupt - they're just not being shared
To share the file, right click on it & then you'll see the option to share file. Click on share file. If the file doesn't change to black right away, refresh the page (using the refresh icon at the top of your browser) or else restart LW.
If a file's corrupt... you'll see a message during download telling you that the file is corrupt. Then you have the choice of going on with the download or killing the download