avformat/mov: do not overwrite extradata in mov_read_glbl()

Fixes regression from Ticket3962

This basically favors the first global header while since 34751f83
it was the last. If this heuristic turns out worse, do not hesitate to revert this and
reopen 3962 as a feature request for multiple STSD
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -1099,6 +1099,10 @@ static int mov_read_glbl(MOVContext *c, AVIOContext *pb, MOVAtom atom)
         if (type == MKTAG('f','i','e','l') && size == atom.size)
             return mov_read_default(c, pb, atom);
     }
+    if (st->codec->extradata_size > 1 && st->codec->extradata) {
+        av_log(c, AV_LOG_WARNING, "ignoring multiple glbl\n");
+        return 0;
+    }
     av_free(st->codec->extradata);
     if (ff_get_extradata(st->codec, pb, atom.size) < 0)
         return AVERROR(ENOMEM);