hi. can you please attach a pcap file that causes this issue.

Certainly! We're you after just a pcap which creates the panic? Do you want that committed somewhere? Or were you more after realistic traffic - although that's going to be a lot harder to find/create/sanitize.

I'm looking to build a test case that fails before this PR and is successful after this PR. at least to know what the corner case actually is :)

Hrmm I have no idea how you actually upload a pcap to GH but we can just use the unit test I added and make a pcap with Python+scapy:

from scapy.all import *
import codecs

pkt = RadioTap(codecs.decode('0000ff002e48000010026c09a000c6070000d4000000881fa1ae9dcbc6304b4b', 'hex'))
wrpcap('panic.pcap', pkt)

Then manually testing with something like:

func main() {
	filePath := flag.String("file", "", "Path to pcap file")
	flag.Parse()

	if *filePath == "" {
		fmt.Println("Provide the path to the pcap file using -file flag")
		os.Exit(1)
	}

	handle, err := pcap.OpenOffline(*filePath)
	if err != nil {
		log.Fatal(err)
	}
	defer handle.Close()

	data, _, err := handle.ZeroCopyReadPacketData()

	if err != nil {
		fmt.Println(err)
	}

	var radioTap layers.RadioTap
	if err := radioTap.DecodeFromBytes(data, gopacket.NilDecodeFeedback); err != nil {
		fmt.Println(err)
	}
	fmt.Println(radioTap.Length)
}

That at least shows the corner case as I set the RadioTap.Length to 255:

$ go run main.go -file ~/panic.pcap
panic: runtime error: slice bounds out of range [255:32]

goroutine 1 [running]:
github.com/gopacket/gopacket/layers.(*RadioTap).DecodeFromBytes(0x38772788?, {0x1f89e50?, 0x6dbd80?, 0x20?}, {0x5ecae8?, 0xc33b20?})
        /home/user/go/pkg/mod/github.com/gopacket/gopacket@v1.2.0/layers/radiotap.go:868 +0xbbc
main.main()
        /home/user/tmp/main.go:37 +0x257
exit status 2

Then obviously with the patch it just runs and shows the Length has been truncated to the frame size:

$ go run main.go -file ~/panic.pcap 
32