Initially, avcodec/srtenc.c was outputting CRLF [1]. Later, a real SRT
muxer was added [2], which outputs LF. The original srtenc.c was
converted to use the muxer [3], changing its output to LF, except for
newline characters within subtitle text.
Fix this to avoid producing SRT files with mixed line endings.
[1] 8e43b6fed9
[2] 9e63c30daa
[3] 55180b3299
Signed-off-by: Kacper Michajłow <kasper93@gmail.com>