border: check ifCurr before dereferencing it

[sgmonroy]

There might be situations where ifCurr has not been set or cannot be
calculated because, ie. malformed headers.

Currently the border router panics in those situations when trying to
create SCMP path offsets information.

Safe check that ifCurr is set before use.

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This change is 

[kormat]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @sgmonroy)

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go/border/rpkt/path.go, line 137 at r1 (raw file):

// instance from the current packet.
func (rp *RtrPkt) mkInfoPathOffsets() scmp.Info {
	if curr, err := rp.IFCurr(); curr == nil || err != nil {

I think perhaps that the rest of the InfoPathOffset could still be created, with the IfID field as 0.

var ifid uint16
if curr, err := rp.IFCurr(); curr != nil && err =!= nil {
	ifid = uint16(curr)
}
...
   IfID ifid, ...

[sgmonroy]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @sgmonroy)

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go/border/rpkt/path.go, line 137 at r1 (raw file):

Previously, kormat (Stephen Shirley) wrote…

I think perhaps that the rest of the InfoPathOffset could still be created, with the IfID field as 0.

var ifid uint16
if curr, err := rp.IFCurr(); curr != nil && err =!= nil {
	ifid = uint16(curr)
}
...
   IfID ifid, ...

But that can be a bit misleading, you don't know if the ifCurr is pointing to a ifid with 0 value or you fail to get the current ifid

[sgmonroy]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @kormat)

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go/border/rpkt/path.go, line 137 at r1 (raw file):

Previously, sgmonroy (Sergio Gonzalez Monroy) wrote…

But that can be a bit misleading, you don't know if the ifCurr is pointing to a ifid with 0 value or you fail to get the current ifid

Done.

[kormat]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[kormat]

Reviewed 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved