Fix GRE header decoding
Fix GRE header decoding

- check if the optional fields exist before consuming data
- `Routing` field is a list of SREs

nhooyr/gopacket,ccordes-snaproute/gopacket,ccordes-snaproute/gopacket,breml/gopacket,LrsK/gopacket,josephcopenhaver/gopacket,breml/gopacket,wheelcomplex/gopacket,UIKit0/gopacket-1,urso/gopacket,breml/gopacket,nhooyr/gopacket,levigross/gopacket,urso/gopacket,aeppert/gopacket,google/gopacket,google/gopacket,levigross/gopacket,ross-williams/gopacket,urso/gopacket,ccordes-snaproute/gopacket,clarktrimble/gopacket,skyportsystems/gopacket,skyportsystems/gopacket,adriantam/gopacket,nhooyr/gopacket,wheelcomplex/gopacket,josephcopenhaver/gopacket,rouzier/gopacket,1d4Nf6/gopacket,hkwi/gopacket,xuzhaokui/gopacket,LrsK/gopacket,1d4Nf6/gopacket,ross-williams/gopacket,LrsK/gopacket,aeppert/gopacket,UIKit0/gopacket-1,rouzier/gopacket,google/gopacket,xuzhaokui/gopacket,skyportsystems/gopacket,zerklabs/gopacket,vtolstov/gopacket,aeppert/gopacket,UIKit0/gopacket-1,clarktrimble/gopacket,1d4Nf6/gopacket,xuzhaokui/gopacket,clarktrimble/gopacket,hkwi/gopacket,zerklabs/gopacket,vtolstov/gopacket,rouzier/gopacket,zerklabs/gopacket,adriantam/gopacket,josephcopenhaver/gopacket,ross-williams/gopacket,vtolstov/gopacket,levigross/gopacket,adriantam/gopacket,hkwi/gopacket,wheelcomplex/gopacket