Panic when calling pes.NewPESHeader

While building a tool to do an analysis of MPEGTS for HLS delivery, ran into a panic while calling pes.NewPESHeader.

```
pes := new(pESHeader)
var err error

if CheckLength(pesBytes, "PES", 6) {

pes.packetStartCodePrefix = uint32(pesBytes[0])<<16 | uint32(pesBytes[1])<<8 | uint32(pesBytes[2])

pes.streamId = uint8(pesBytes[3])
pes.pesPacketLength = uint16(pesBytes[4])<<8 | uint16(pesBytes[5])
---> pes.dataAlignment = pesBytes[6]&0x04 != 0
dataStartIndex := 6
```

The code panics on the pesBytes[6] check being out of bounds.

I think that CheckLength should be called with a value of seven in this case since it's checking the length of the byte array which is one-based, but the index is zero based.

Thank you for this code, it made getting started much easier!