Made the memcmp address sanitizer clean.

Added extra check to ensure memcmp gets called with a size that is
guaranteed within range of the buffer. This wasn't a real problem,
but stops address sanitizer from complaining.

See:
https://github.com/google/flatbuffers/issues/88
https://github.com/joker-eph/flatbuffers/commit/517506b4e105814e0823353c3d6561085c26a1ab#commitcomment-8265231

Change-Id: I7de24da2d36d973e154f92eeb3e093070886037f
Tested: on Linux