Work around leak in boost flat_set

Summary: This is very likely to be the memory leak reported to be new in
HHVM 3.4. See code comment and linked GitHub issue for full explanation.

Fixes https://github.com/facebook/hhvm/issues/4268

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Reviewed By: @paulbiss

Differential Revision: D1736543
releasing 3.4.1
targeting 3.4.2