Fix build when using -Werror=undef

[bklarson]

Correct a couple places where macros were being checked when they
might not exist in some cases.

Fixes #3356.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[bazel-io]

Can one of the admins verify this patch?

[googlebot]

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[bklarson]

I've (just now) signed the CLA.

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[googlebot]

CLAs look good, thanks!

[acozzette]

ok to test (this will trigger the other CI runs)

[acozzette]

@bklarson Thanks for the pull request!