afpacket.(*TPacket).ZeroCopyReadPacketData segmentation violation

Hi there!

I periodically get the following error when using `afpacket.(*TPacket).ZeroCopyReadPacketData`:

```
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x1 addr=0x7f5e7bc10008 pc=0x900029]

goroutine 66 [running]:
runtime.throw(0xa676ad, 0x5)
runtime/panic.go:1117 +0x72 fp=0xc0005f7cb0 sp=0xc0005f7c80 pc=0x4378f2
runtime.sigpanic()
runtime/signal_unix.go:741 +0x268 fp=0xc0005f7ce8 sp=0xc0005f7cb0 pc=0x44e3a8
github.com/google/gopacket/afpacket.(*v3wrapper).getStatus(0xc000128dc0, 0x1)
github.com/google/gopacket@v1.1.20-0.20210304165259-20562ffb40f8/afpacket/header.go:160 +0x9 fp=0xc0005f7cf0 sp=0xc0005f7ce8 pc=0x900029
github.com/google/gopacket/afpacket.(*TPacket).pollForFirstPacket(0xc000128d00, 0xb8e800, 0xc000128dc0, 0x38, 0x48)
github.com/google/gopacket@v1.1.20-0.20210304165259-20562ffb40f8/afpacket/afpacket.go:453 +0x5e fp=0xc0005f7d48 sp=0xc0005f7cf0 pc=0x8ff6fe
github.com/google/gopacket/afpacket.(*TPacket).ZeroCopyReadPacketData(0xc000128d00, 0xc00067e820, 0x1, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
github.com/google/gopacket@v1.1.20-0.20210304165259-20562ffb40f8/afpacket/afpacket.go:302 +0x147 fp=0xc0005f7de0 sp=0xc0005f7d48 pc=0x8fe747

```

Possibly related to https://github.com/google/gopacket/issues/717