avformat/mov: Mark avio context of decompressed atoms as seekable

Fixes Ticket4329
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -3344,6 +3344,7 @@ static int mov_read_cmov(MOVContext *c, AVIOContext *pb, MOVAtom atom)
         goto free_and_return;
     if (ffio_init_context(&ctx, moov_data, moov_len, 0, NULL, NULL, NULL, NULL) != 0)
         goto free_and_return;
+    ctx.seekable = AVIO_SEEKABLE_NORMAL;
     atom.type = MKTAG('m','o','o','v');
     atom.size = moov_len;
     ret = mov_read_default(c, &ctx, atom);