Fix bug #18036 with infloop in redisplay with huge fringes.

 src/xdisp.c (move_it_vertically_backward, move_it_by_lines): Prevent
 infinite looping in redisplay when display lines don't have enough
 space to display even a single character.

src/ChangeLog

+2014-07-16  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (move_it_vertically_backward, move_it_by_lines): Prevent
+	infinite looping in redisplay when display lines don't have enough
+	space to display even a single character.  (Bug#18036)
+
 2014-07-13  Eli Zaretskii  <eliz@gnu.org>
 
 	* xdisp.c (decode_mode_spec): Call file-remote-p on the current

src/xdisp.c

@@ -9351,7 +9351,7 @@ move_it_vertically_backward (struct it *it, int dy)
   /* Estimate how many newlines we must move back.  */
   nlines = max (1, dy / default_line_pixel_height (it->w));
-  if (it->line_wrap == TRUNCATE)
+  if (it->line_wrap == TRUNCATE || nchars_per_row == 0)
     pos_limit = BEGV;
   else
     pos_limit = max (start_pos - nlines * nchars_per_row, BEGV);
@@ -9606,7 +9606,7 @@ move_it_by_lines (struct it *it, ptrdiff_t dvpos)
       /* Go back -DVPOS buffer lines, but no farther than -DVPOS full
 	 screen lines, and reseat the iterator there.  */
       start_charpos = IT_CHARPOS (*it);
-      if (it->line_wrap == TRUNCATE)
+      if (it->line_wrap == TRUNCATE || nchars_per_row == 0)
 	pos_limit = BEGV;
       else
 	pos_limit = max (start_charpos + dvpos * nchars_per_row, BEGV);