Fix nested extensions crash p2

[lnicola]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to CHANGELOG.md if knowledge of this change could be valuable to users.

Closes #82

Alternative to #82 with the simplification I proposed there. Not squashed to preserve attribution.

[lnicola]

r? anyone?

[urschrei]

Bors r=lnicola

[bors[bot]]

Build failed:

• ci result

[lnicola]

bors try

[bors[bot]]

try

Build failed:

• ci result

[lnicola]

bors r=urschrei

[bors[bot]]

Build failed:

• ci result

[lnicola]

bors r=urschrei

[bors[bot]]

Build succeeded!

The publicly hosted instance of bors-ng is deprecated and will go away soon.

If you want to self-host your own instance, instructions are here. For more help, visit the forum.

If you want to switch to GitHub's built-in merge queue, visit their help page.

• ci result