flat text DB: fix move by bookmark

To test whether the range [a,b( is *before* e,
compare e to b, not to a.

This makes a difference when a==b, which happens for row 0 (header)
when there is no header.

Conflicts:
	connectivity/source/drivers/flat/ETable.cxx

Change-Id: I629b71936f82a468febe0360909264dd80304437
Reviewed-on: https://gerrit.libreoffice.org/6866
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Eike Rathke <erack@redhat.com>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>