runtime: skip zero-sized fields in structs when converting to FFI

runtime: skip zero-sized fields in structs when converting to FFI

    The libffi library doesn't understand zero-sized objects.
    When we see a zero-sized field in a struct, just skip it when
    converting to the FFI data structures. There is no value to pass in
    any case, so not telling libffi about the field doesn't affect
    anything.

    The test case for this is https://golang.org/cl/123316.

    Fixes golang/go#26335

    Reviewed-on: https://go-review.googlesource.com/123335

From-SVN: r262651