folly: avoid used-uninitialized bug in a test

folly: avoid used-uninitialized bug in a test

Summary:
Clang complained about this.
* folly/gen/test/FileBenchmark.cpp (BENCHMARK): Initialize "rfd" to -1
and verify that it is set to something nonnegative in the first
BENCHMARK_SUSPEND block.

Test Plan:
Run this:
fbconfig -r --clang --with-project-version clang:dev folly/gen && fbmake runtests
and observe that compilation now succeeds. (and test results remain unchanged)

Reviewed By: tudorb@fb.com

Subscribers: njormrod

FB internal diff: D1551180

Tasks: 4090011