Subtitles: Remove extraneous character in embedded text subtitles.

The NUL character at the end of the C string was accidentally being
included as part of the subtitle string, and rendered to the screen.
This affected only "raw text" subtitles directly embedded into the
video.  Fixes #11855.
(cherry picked from commit b062dd43ba26836c854a0c8fda845f9997b8fba9)