avformat/matroskaenc: Fix use of uninitialized value

Regression since 67eea6cf02.
Affects only WebVTT when muxing WebM. (This is covered
by the webm-webvtt-remux FATE test which fails for several
FATE boxes on fate-ffmpeg.org.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/matroskaenc.c

@@ -1914,6 +1914,7 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
     }
 
     end_ebml_master(pb, track_master);
+    ret = 0;
 fail:
     ffio_reset_dyn_buf(mkv->tmp_bc);