Summary:
The calculation was too conservative, and could result in copying zero
frames into the new stack chunk, which caused a knock-on failure in
the interpreter.

Test Plan: Tested on an in-house repro (not shareable, unfortunately)

Reviewers: niteria, bgamari, austin, erikd

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4052