Pull out a local variable in MMapAlloc
Pull out a local variable in MMapAlloc

Summary:
[Folly] Pull out a local variable in `MMapAlloc`.

Moves the conditional compilation out of a compilcated expression and into a separate full-statement by itself.

Reviewed By: Orvid

Differential Revision: D9598437

fbshipit-source-id: 770c835f729e72b880aadcefb8aae7ead2754bd9

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