nanobench: Add some Valgrind failure paranoia.
nanobench: Add some Valgrind failure paranoia.

Trying to figure out

http://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-Valgrind/builds/586/steps/nanobench/logs/stdio

BUG=skia:4482

Review URL: https://codereview.chromium.org/1418633002

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