vc1_parser: fix NULL pointer dereference

Fixes Ticket456

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

libavcodec/vc1.c

@@ -896,6 +896,8 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
     v->rnd = get_bits1(gb);
     if (v->interlace)
         v->uvsamp = get_bits1(gb);
+    if(!ff_vc1_bfraction_vlc.table)
+        return 0; //parsing only, vlc tables havnt been allocated
     if (v->field_mode) {
         if (!v->refdist_flag)
             v->refdist = 0;