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.

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>

1 files changed, 13 insertions, 0 deletions

diff --git a/libavformat/subtitles.h b/libavformat/subtitles.h
index 455b374f25..8f68e7bda1 100644
--- a/libavformat/subtitles.h
+++ b/libavformat/subtitles.h
@@ -96,4 +96,17 @@ const char *ff_smil_get_attr_ptr(const char *s, const char *attr);
  */
 void ff_subtitles_read_chunk(AVIOContext *pb, AVBPrint *buf);
 
+/**
+ * Get the number of characters to increment to jump to the next line, or to
+ * the end of the string.
+ */
+static av_always_inline int ff_subtitles_next_line(const char *ptr)
+{
+    int n = strcspn(ptr, "\n");
+    ptr += n;
+    if (*ptr == '\n')
+        n++;
+    return n;
+}
+
 #endif /* AVFORMAT_SUBTITLES_H */