fix unused result warning

[conradsnicta]

As per #1373

[rcurtin]

Thanks, I'll go ahead and merge this when #1373 is merged.

Do you want to add your name to the list of contributors? (or I can do it for you if you like) Even though it is a simple change it is still a change to the code.

We also mail mlpack stickers to contributors so if you want I can toss some in an envelope and send them to you...

[zoq]

Thanks for the backport, no comments from my side.

[conradsnicta]

@rcurtin - if the contribution is sufficient, then adding to the list of contributors sounds good (but please don't include my email address).

I'll have to make more contributions before deserving the stickers :)

[rcurtin]

Sure, added in 46eb0a0 with no email. I have international stamps so it is easy for me to mail stickers, so just let me know if you ever want any. :)