storageAddCapabilities: fix bug in updating nursery pointers

Authored by simonmar on Apr 27 2018, 1:31 PM.

Description

storageAddCapabilities: fix bug in updating nursery pointers

Summary:
We were unconditionally updating the nursery pointers to be
nurseries[cap->no], but when using nursery chunks this might be
wrong. This manifested as a later assertion failure in allocate().

Test Plan: new test case

Reviewers: bgamari, niteria, erikd

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4649