if we move to the front of a segment, use the "slow" technique for moving

if we move to the front of a segment, use the "slow" technique for moving

2001-10-24  Havoc Pennington  <hp@pobox.com>

	* gtk/gtktextiter.c (gtk_text_iter_backward_chars): if we move to
	the front of a segment, use the "slow" technique for moving
	backward, so we don't mess up the any_segment field in the iter.
	Fixes #57707

	* tests/testtextbuffer.c: add code to detect #57707, to avoid
	regression