avformat/subtitles: Honour ff_subtitles_read_line() documentation

It claims to always zero-terminate its buffer like snprintf(),
yet it does it not on EOF. Because of this the mcc demuxer
used uninitialized values when reading an empty input file.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit 8766361fc1)

libavformat/subtitles.c

@@ -418,6 +418,7 @@ ptrdiff_t ff_subtitles_read_line(FFTextReader *tr, char *buf, size_t size)
     size_t cur = 0;
     if (!size)
         return 0;
+    buf[0] = '\0';
     while (cur + 1 < size) {
         unsigned char c = ff_text_r8(tr);
         if (!c)