avoid panic when counting zero-sized outputs in count()

[Shadow53]

Prerequisites

Resolves #1617.

• Added a test case to tests/issues.rs
• I also annotated an unrelated test-looking function with #[test] since it looked like it should be marked as such

Code style

This project follows a code style
checked by [rustfmt][https://github.com/rust-lang-nursery/rustfmt].

Please avoid cosmetic fixes unrelated to the pull request. Keeping the changes
as small as possible increase your chances of getting this merged quickly.

There were a couple of files that were modified by cargo fmt that I did not edit, so I undid those changes.

[Geal]

Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 3880565977

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 62.485%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/multi/mod.rs	1	2	50.0%

Totals
Change from base Build 3863115493:	0.02%
Covered Lines:	1559
Relevant Lines:	2495

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💛 - Coveralls