This fixes a bunch of possible overread in avformat with the idiom p +=
strcspn(p, "\n") + 1 (strcspn() can focus on the trailing '\0' if no
'\n' is found, so the +1 leads to an overread).
Note on lavf/matroskaenc: no extra subtitles.o Makefile dependency is
added because only the header is required for ff_subtitles_next_line().
Note on lavf/mpsubdec: code gets slightly complex to avoid an infinite
loop in the probing since there is no more forced increment.
NOTE:
Code of function ff_subtitles_next_line fixed by Alexander Strasser.
The original code from master did test the wrong character, but was
corrected by a subsequent commit. That commit however is not backported,
so it had to be fixed in this commit for the backport.
Conflicts:
libavformat/mpl2dec.c
(cherry picked from commit 90fc00a623de44e137fe1601b91356e8cd8bdd54)
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
