In STRICT mode, out-of-bounds values caused an error message
to be queued (rather than just a warning), without any further
error-like processing happening. (The error is queued during
update, at which time it's too late. For it to be processed
properly, it would need to be queued during check-stage.)
The assertion rightfully complains that we're trying to send
an OK while having an error queued.

Changeset breaks a lot of tests out into check-stage. This also
allows us to send more correct warnings/error messages.

sql/set_var.cc:
  cleanup: fold get_unsigned() and fix_unsigned() into one,
  as well as all the semi-common code from the ::check
  functions.