Problem:
For some queries where sorting/grouping require a temporary table,
we might actually end up with a temporary table without any columns.
That means that the 'read_set' 'tmp_set' and 'cond_set' bitmaps for
the temporary table are zero-length, which is not supported by our
bitmap implementation. If such a temporary table is passed on to
filesort() for sorting, we may hit undefined behaviour, because
we access the bits of a zero-length bitmap.

Solution:
Handle zero-length bitmaps in filesort().
If the bitmap is empty, we are only sorting constant expressions
anyways, and the read-set of the temporary table will be empty.

In the bitmap implementation:
Make it clear that a bitmap of size zero can be initialized,
but cannot be used. Add more DBUG_ASSERTs to ensure that no
bits are accessed. The only exception being bitmap_init
(which initializes the bitmap struct)
and bitmap_clear_all/bitmap_set_all (which are no-ops)