[#180208] more careful _abort_write_group for RepositoryPackCollection

[Martin Pool]

In the bug report <https://bugs.edge.launchpad.net/bzr/+bug/180208> it
seems that sometimes there is no new_pack when the repository is in a
write group.  Usually there should be, but if an error occurs at an
unexpected time it's possible.

-- 
Martin
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