avformat/subtitles: add a next line jumper and use it.

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.

1 files changed, 1 insertions, 1 deletions

diff --git a/libavformat/microdvddec.c b/libavformat/microdvddec.c
index 7254bcf7f8..7b49e43976 100644
--- a/libavformat/microdvddec.c
+++ b/libavformat/microdvddec.c
@@ -47,7 +47,7 @@ static int microdvd_probe(AVProbeData *p)
             sscanf(ptr, "{%*d}{%*d}%c",  &c) != 1 &&
             sscanf(ptr, "{DEFAULT}{}%c", &c) != 1)
             return 0;
-        ptr += strcspn(ptr, "\n") + 1;
+        ptr += ff_subtitles_next_line(ptr);
     }
     return AVPROBE_SCORE_MAX;
 }