Two new juicy assertions in the SQL front-end

Dear MonetDB developers,

I'm reporting two other assertions I hit in the front-end. The first is related to scalar subqueries, see query3.sql for a repro and backtrace3.txt. I guess it could be rather considered a feature request... BTW I appreciate this style of treating unsupported features, I'm starting to use in my changes as well :)

The second occurs somewhere in the query rewriter when issuing the whole query, see query4.sql and backtrace4.txt.

On Linux, tip of the default branch, debug build, empty tables, schema attached.

Thanks, Dean