Removing compiler warning

[syclik]

Submisison Checklist

• Run unit tests: ./runTests.py test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary:

Squashes a compiler warning.

How to Verify:

Run:
./runTests.py test/unit/math/fwd/arr/meta/OperandsAndPartials_test.cpp

There should be no compiler warnings. In the current develop branch, there is a warning.

Side Effects:

This isn't actually used in Stan yet, so there should be no side-effects.

Documentation:

None required.

Reviewer Suggestions:

Anyone.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Columbia University

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bob-carpenter]

Looks good.