Fix some minor errors in new PHJ code.

Fix some minor errors in new PHJ code.

Correct ExecParallelHashTuplePrealloc's estimate of whether the
space_allowed limit is exceeded.  Be more consistent about tuples that
are exactly HASH_CHUNK_THRESHOLD in size (they're "small", not "large").
Neither of these things explain the current buildfarm unhappiness, but
they're still bugs.

Thomas Munro, per gripe by me

Discussion: https://postgr.es/m/CAEepm=34PDuR69kfYVhmZPgMdy8pSA-MYbpesEN1SR+2oj3Y+w@mail.gmail.com