Update an sbuf assertion to allow for the new SBUF_INCLUDENUL flag. If
INCLUDENUL is set and sbuf_finish() has been called, the length has been
incremented to count the nulterm byte, and in that case current length is
allowed to be equal to buffer size, otherwise it must be less than.

Add a predicate macro to test for SBUF_INCLUDENUL, and use it in tests, to
be consistant with the style in the rest of this file.