Fix MSVC build for SimpleFixedString
Fix MSVC build for SimpleFixedString

Summary:
I don't know why MSVC doesn't like the ternary here, but this isn't obviously
a reduction in code quality, so let's just do it.

Reviewed By: passy

Differential Revision: D23820253

fbshipit-source-id: 5b494e9f969985c87c6b6dd1711cd935cd415b70

facebookincubator/fbjni,facebookincubator/fbjni,facebookincubator/fbjni,facebookincubator/fbjni