coll: fix allgatherv early return case

[hzhou]

Pull Request Description

The localcopy omitted considering displs.

Thanks @mathstuf for discovering the bug. ref: #5034 (comment)

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[hzhou]

@mathstuf Could you verify the fix?

[mathstuf]

Thanks, will test. Here's a test case I adapted (but haven't run because I built MPICH out-of-source and test/mpi doesn't seem to be set up for that…): mathstuf@1935ffe

[mathstuf]

That works!

[mathstuf]

Backport for Fedora requested: https://bugzilla.redhat.com/show_bug.cgi?id=2162892