erspan: do not assume transport header is always set
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.4.202 by commit 02da602bc2f353dccd9e489a604490034ded941e, it was introduced in version v4.18 by commit d5db21a3e6977dcb42cee3d16cd69901fa66510a. For more details please see the references link.