…nability and robustness (#215)

This is a step towards improving the transfer reassembler.

This changeset also renames `redundant_transport_index` as
`redundant_iface_index` for consistency with the other implementations;
this change is not visible at the API level.

…c7022'

git-subtree-dir: src/libcanard
git-subtree-mainline: acc1a0d
git-subtree-split: 644d3f7