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, 2 insertions, 1 deletions

diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index 7b69bc3abf..2c128ccbb7 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -28,6 +28,7 @@
 #include "isom.h"
 #include "matroska.h"
 #include "riff.h"
+#include "subtitles.h"
 #include "wv.h"
 
 #include "libavutil/avstring.h"
@@ -1363,7 +1364,7 @@ static int srt_get_duration(uint8_t **buf)
             s_hsec += 1000*s_sec;       e_hsec += 1000*e_sec;
             duration = e_hsec - s_hsec;
         }
-        *buf += strcspn(*buf, "\n") + 1;
+        *buf += ff_subtitles_next_line(*buf);
     }
     return duration;
 }