wtvdec: fix null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/wtvdec.c

@@ -763,7 +763,7 @@ static int parse_chunks(AVFormatContext *s, int mode, int64_t seekts, int *len_p
             }
         } else if (!ff_guidcmp(g, ff_stream2_guid)) {
             int stream_index = ff_find_stream_index(s, sid);
-            if (stream_index >= 0 && !((WtvStream*)s->streams[stream_index]->priv_data)->seen_data) {
+            if (stream_index >= 0 && s->streams[stream_index]->priv_data && !((WtvStream*)s->streams[stream_index]->priv_data)->seen_data) {
                 ff_asf_guid mediatype, subtype, formattype;
                 int size;
                 avio_skip(pb, 12);