Avoid assertion violations in 'back_to_previous_visible_line_start'

Avoid assertion violations in 'back_to_previous_visible_line_start'

* src/xdisp.c (init_iterator): Always initialize narrowed_begv to
zero, since SET_WITH_NARROWED_BEGV depends on it being non-zero as
an indication that long-line optimizations are in use.
(back_to_previous_visible_line_start): When long-line
optimizations are in effect, we may end up not on a newline.