Summary:
The fix is a bit clunky, and is perhaps not the best fix, but I'm not
sure how much work it would be to fix it the other way (see comments
for more info).

Test Plan: T7919 doesn't crash

Reviewers: austin, rwbarton, ezyang, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1113

GHC Trac Issues: #7919