Fix IndexError/struct.error when scanning IPv6 Hop-by-Hop header for a Jumbo option

[AbdaullahAG]

Description

IPv6.extract_padding looks for a Jumbo option in the Hop-by-Hop header with while offset <= len(data). When no Jumbo option is found and len(data) equals an offset visited by the loop, data[offset] raises IndexError. When an 0xc2 type byte is found with fewer than 6 bytes remaining, struct.unpack raises struct.error.

scapy/scapy/layers/inet6.py

Lines 366 to 370 in 7daa15d

	offset = 4 * idx + 2
	while offset <= len(data):
	opt_type = data[offset]
	if opt_type == 0xc2: # Jumbo option
	jumbo_len = struct.unpack("I", data[offset + 2:offset + 2 + 4])[0] # noqa: E501

from scapy.all import IPv6

hdr = bytes(IPv6(nh=0, plen=0))
IPv6(hdr + b"\x3b\x00\x01\x00\x00\x00\x00\x00\x00\x00")  # IndexError
IPv6(hdr + b"\x3b\x00\x01\x00\x00\x00\xc2\x04\x00\x00")  # struct.error

The loop now requires 6 bytes (type, length, 4-byte value) to remain. A regression test is added to test/scapy/layers/inet6.uts.