Extend sshbuf validation

Extend sshbuf validation

Multiple sshbuf structs can be linked through a parent/child relationship.
Make sure that a single sshbuf cannot be its own parent. If this would ever
happen, it would result in reference counting issues.

This is a cheap way of testing this with very little overhead. It does not
detect A->B->A linkages though for performance reason and the fact that it
takes a programming error for this to occur anyway.

Authored with Benny Baumann (BenBE at geshi dot org).

ok djm@