Fix invalid-shift-base in thrift/test/VarintTest.cpp
Fix invalid-shift-base in thrift/test/VarintTest.cpp

Summary: [Thrift] Fix `invalid-shift-base` in `thrift/test/VarintTest.cpp` (`-fsanitize=shift`).

Reviewed By: igorsugak

Differential Revision: D6928384

fbshipit-source-id: 3fd18920f332e31378dac6a65646d3b9f8a6c834

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