This provides higher precission
Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

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.

This should make probing faster.

Demuxers where the changed code looked less readable where skiped
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>