Make AllocSetContextCreate throw an error for bad context-size parameters.

Make AllocSetContextCreate throw an error for bad context-size parameters.

The previous behavior was to silently change them to something valid.
That obscured the bugs fixed in commit ea268cdc9, and generally seems
less useful than complaining.  Unlike the previous commit, though,
we'll do this in HEAD only --- it's a bit too late to be possibly
breaking third-party code in 9.6.

Discussion: <CA+TgmobNcELVd3QmLD3tx=w7+CokRQiC4_U0txjz=WHpfdkU=w@mail.gmail.com>