helpers/memenv/memenv.cc used SIZE_MAX without including <stdint.h>.
Since we're fixing this problem, replace SIZE_MAX with
std::numeric_limits<size_t>::max(), which is clearer.

Fixes https://github.com/google/leveldb/issues/562

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189821707