Do not mess with NDEBUG in fbstring

Summary: Temporarily overriding `NDEBUG` can have unexpected side-effects depending on the implementation of `<assert.h>`. Better use an explicit macro for `assert()` that we can explicitly disable.

Reviewed By: Gownta

Differential Revision: D3850717

fbshipit-source-id: b1e7fce89ab4120b0224e9c6bf668d983b3d2d31