Fix an ASan issue in cost_test.cc

[m-chaturvedi]

To reproduce: bazel test --config=asan --copt -O0 //solvers:cost_test

Error
Reference
Build failure

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This change is 

[m-chaturvedi]

@drake-jenkins-bot mac-mojave-clang-bazel-experimental-address-sanitizer-release please

[jamiesnape]

To clarify, the mac-mojave-clang-bazel-experimental-address-sanitizer failed build is expected, but the number of failed tests in that build has decreased by one?

[EricCousineau-TRI]

, given Jamie's clarification. (Can't think of how that wouldn't be the case...)

Reviewable status: needs at least two assigned reviewers (waiting on @EricCousineau-TRI)

[m-chaturvedi]

To clarify, the mac-mojave-clang-bazel-experimental-address-sanitizer failed build is expected, but the number of failed tests in that build has decreased by one?

Sorry for the delay in replying.
Yes, the the number of failures decrease from 3 to 2.

//solvers:cost_test passes, //math:normalize_vector_test and //systems/rendering:pose_aggregator_test still fail.

[m-chaturvedi]

+@SeanCurtis-TRI for platform review, please.

[SeanCurtis-TRI]

On the basis that the changes seem, at the very least, harmless. I'll buy the "errors are getting better"-ness of this PR.

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all discussions resolved, LGTM from assignees SeanCurtis-TRI(platform),EricCousineau-TRI(platform) (waiting on @EricCousineau-TRI)