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.

libavformat/jacosubdec.c

@@ -63,7 +63,7 @@ static int jacosub_probe(AVProbeData *p)
                 return AVPROBE_SCORE_EXTENSION + 1;
             return 0;
         }
-        ptr += strcspn(ptr, "\n") + 1;
+        ptr += ff_subtitles_next_line(ptr);
     }
     return 0;
 }