Assert check which is related to a segfault I sometimes see with matroska

I had problems with corrupt AvPacket object buf pointers in the avbin project. This is based on an older version of libav 46a86c61940ec8efdfa90b23f05c38ecc1184a34. 

The media is VP8 video in a matroska container. A segfault occurs in vp8_parser.c line 30:

```c
s->pict_type= (buf[0]&0x01) ? AV_PICTURE_TYPE_P : AV_PICTURE_TYPE_I;
```

buf is NULL. I traced the problem as far as the corrupt AvPacket. I am not sure anyone wants this patch, but it might make testing easier.