avformat/realtextdec: Check for duplicate extradata in realtext_read_header()

Fixes: memleak
Fixes: 16140/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5684008052064256

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 652ea23cb3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/realtextdec.c

@@ -87,6 +87,10 @@ static int realtext_read_header(AVFormatContext *s)
             /* save header to extradata */
             const char *p = ff_smil_get_attr_ptr(buf.str, "duration");
 
+            if (st->codecpar->extradata) {
+                res = AVERROR_INVALIDDATA;
+                goto end;
+            }
             if (p)
                 duration = read_ts(p);
             st->codecpar->extradata = av_strdup(buf.str);