Use of GetFabricIndex was broken in session handle refactor

[bzbarsky-apple]

Problem

#13330 replaced code like this:

    return mpExchangeCtx->GetSessionHandle().GetFabricIndex();

with code like this:

    return mpExchangeCtx->GetSessionHandle()->AsSecureSession()->GetFabricIndex();

at a large number of callsites.

Unfortunately, the latter will crash if the session is a group session, whereas the former worked correctly in that case.

Proposed Solution

Fix things so we don't crash if someone groupcasts a command that cares about the fabric index. In practice this probably means moving mFabricIndex up to Session, having it initialized to 0 for UnauthenticatedSession, initializing it correctly for group and secure sessions, and implementing GetFabricIndex on Session. Or is there a reason that can't happen?

@kghost @andreilitvin @msandstedt

[msandstedt]

Proposed Solution

Fix things so we don't crash if someone groupcasts a command that cares about the fabric index. In practice this probably means moving mFabricIndex up to Session, having it initialized to 0 for UnauthenticatedSession, initializing it correctly for group and secure sessions, and implementing GetFabricIndex on Session. Or is there a reason that can't happen?

I think this makes sense. Fabric index 0 is our special reserved value for no-fabric, so it's exactly for this type of thing.