Filesort temporarily changes read_set to be tmp_set and marks only
fields needed for filesort. Add an assert to ensure that it doesn't
overwrite the old value of tmp_set, that is that read_set was *not*
already tmp_set when filesort was invoked.

Fix sql_update.cc that was was doing exactly that - changing read_set to
tmp_set, configuring tmp_set for keyread, and then invoking filesort.