Add check if 'link' in options is instance of dict.

[mielvds]

An empty dict is not truthy in python, so cannot just check
for "if options['link']"

Fixes #64
Replaces #66 because it was too outdated for a merge.

[mielvds]

This PR introduces a regression in the framing implementation. While the use of options['link'] is error-prone, the current code somehow relies on this side-effect. Further investigation is needed before this becomes an actual fix.