avformat/avidec: Check nb_streams in read_gab2_sub()

Fixes null pointer dereference
Fixes: 1/null_point.avi
Found-by: 连一汉 <lianyihan@360.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2679ad47)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avidec.c

@@ -1097,6 +1097,8 @@ static int read_gab2_sub(AVFormatContext *s, AVStream *st, AVPacket *pkt)
             goto error;
 
         if (!avformat_open_input(&ast->sub_ctx, "", sub_demuxer, NULL)) {
+            if (ast->sub_ctx->nb_streams != 1)
+                goto error;
             ff_read_packet(ast->sub_ctx, &ast->sub_pkt);
             avcodec_parameters_copy(st->codecpar, ast->sub_ctx->streams[0]->codecpar);
             time_base = ast->sub_ctx->streams[0]->time_base;