Handle small_vectors with 0 inline capacity correctly

Handle small_vectors with 0 inline capacity correctly

Summary: It is an error to attempt to get the size of a zero length array, such as `unsigned char[0]`, which is what you get if `MaxInline` has been passed in as 0. We can work around this by simply defining `InlineStorageType` to be `void*` if the size is exactly 0, which will result in the capacity correctly being stored out of line.

Reviewed By: yfeldblum

Differential Revision: D3572898

fbshipit-source-id: c96bb7cc6a890044bb74b0f6d5238c503552ee25