strengthened conditions for using workplace into fse table space

strengthened conditions for using workplace into fse table space

ensure that the structure layout is as expected.
will trigger an error if it changes in the future.

Another solution would be to use a union,
this would be cleaner and get rid of these static asserts.

However, in order to keep the current code unmodified,
it would be necessary to use an un-named unions.
And apparently, un-named unions are only possible on "recent" compilers (C99+).