avformat/realtextdec: free queue on error

Fixes: memleak
Fixes: 16277/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5696629440512000

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 493438fafc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/realtextdec.c

@@ -123,6 +123,8 @@ static int realtext_read_header(AVFormatContext *s)
 
 end:
     av_bprint_finalize(&buf, NULL);
+    if (res < 0)
+        ff_subtitles_queue_clean(&rt->q);
     return res;
 }