Fix bug #18584 with assertion violations while scrolling.

 src/xdisp.c (move_it_by_lines): Call reseat_1 after moving the
 iterator backwards, to resync the bidi iterator.

src/ChangeLog

+2014-10-02  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (move_it_by_lines): Call reseat_1 after moving the
+	iterator backwards, to resync the bidi iterator.  (Bug#18584)
+
 2014-10-01  Jan Djärv  <jan.h.d@swipnet.se>
 
 	* xfaces.c (Finternal_set_lisp_face_attribute): Don't try to

src/xdisp.c

@@ -9748,6 +9748,7 @@ move_it_by_lines (struct it *it, ptrdiff_t dvpos)
 	      back_to_previous_visible_line_start (it);
 	      it->vpos--;
 	    }
+	  reseat_1 (it, it->current.pos, 1);
 	}
       else
 	RESTORE_IT (it, it, it2data);