rts: Fix uninitialised variable uses

Strangely gcc 5.4 compiling on amd64 (nixos) complained about these.
Both warnings look correct, so I'm not sure why we haven't been seeing
these up until now.

Test Plan: Validate

Reviewers: simonmar, austin, erikd

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3693