lavf/webm_chunk: Respect buffer size

The last argument of av_strlcpy is supposed to contain the size of the
destination buffer, but it was filled with the size of the source
string, effectively negating its very purpose.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 73ef1f47f5)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/webm_chunk.c

@@ -93,7 +93,7 @@ static int get_chunk_filename(AVFormatContext *s, int is_header, char *filename)
         if (!wc->header_filename) {
             return AVERROR(EINVAL);
         }
-        av_strlcpy(filename, wc->header_filename, strlen(wc->header_filename) + 1);
+        av_strlcpy(filename, wc->header_filename, MAX_FILENAME_SIZE);
     } else {
         if (av_get_frame_filename(filename, MAX_FILENAME_SIZE,
                                   s->filename, wc->chunk_index - 1) < 0) {