Squash warning about possibly-uninitialized variable
Squash warning about possibly-uninitialized variable

Summary: `mentry` can only be uninitialized on a path that shouldn't execute.

Reviewed By: int3

Differential Revision: D3526060

fbshipit-source-id: 1059f648d6d349493ba2c23e70c1990a3a7af5f8

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