NewPESHeader causes memory escape

Version of go
```
go version go1.12 linux/amd64
```

Example:
```
package main

import (
"encoding/hex"

"github.com/Comcast/gots/packet"
"github.com/Comcast/gots/pes"
)

func main() {
pkt := parseHexString(
"4740661a000001c006ff80800521dee9ca57fff94c801d2000210995341d9d43" +
"61089848180b0884626048901425ddc09249220129d2fce728111c987e67ecb7" +
"4284af5099181d8cd095b841b0c7539ad6c06260536e137615560052369fc984" +
"0b3af532418b3924a6b28d6208a6a9e3d22d533ec89646246c734a696407a95e" +
"3bf230404a4ad0000201038cf0c6a2e32abda45b7effe9f79280a137ed120fd3" +
"bd8252e07cddadbe6d2b60084208500e06f6ceb6acf2c43011c5938b")

test(&pkt)
}

func test(pkt *packet.Packet) {
data, err := packet.PESHeader(pkt)
if err != nil {
panic(err)
}

pesHeaderData := make([]byte, packet.PacketSize)
copy(pesHeaderData, data)

_, err = pes.NewPESHeader(pesHeaderData[:len(data)])
if err != nil {
panic(err)
}
}

func parseHexString(h string) packet.Packet {
b, err := hex.DecodeString(h)
if err != nil {
panic("bad test: " + h)
}
var pkt packet.Packet
if copy(pkt[:], b) != packet.PacketSize {
panic("bad test (wrong length): " + h)
}

return pkt
}
```
```
go build --gcflags '-m -l' escape.go
# command-line-arguments
./escape.go:40:22: "bad test: " + h escapes to heap
./escape.go:44:37: "bad test (wrong length): " + h escapes to heap
./escape.go:37:21: parseHexString h does not escape
./escape.go:43:13: parseHexString pkt does not escape
./escape.go:28:23: make([]byte, packet.PacketSize) escapes to heap
./escape.go:22:11: test pkt does not escape
./escape.go:19:7: main &pkt does not escape
```

Solution options:
- Use sync.Pool for pESHeader.data
- Use copy of pesBytes for pESHeader.data
- Something else?