SSA subtitles rendering: Improve error checking to avoid possible crashes.

Some crashes could occur when the SSA script was using obviously wrong values.
Replace "On top > Never" by "On top > Default".

In most of the case "On top > Default" will have the exact same behavior than "On top > Never" but it won't try to override the "on top" flag if an external tool sets it.

This commit fixes #2689.