Avoid indexing on LegacyBuildGraph.reset().
Avoid indexing on LegacyBuildGraph.reset().

Avoid indexing on `LegacyBuildGraph.reset()`.

Fixes #3389.

Testing Done:
Tested against the repro in #3389 + https://travis-ci.org/pantsbuild/pants/builds/129541480

Bugs closed: 3389, 3404

Reviewed at https://rbcommons.com/s/twitter/r/3868/

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