Fix bug #9955 with mouse-highlight that starts at beginning of window.

 src/xdisp.c (note_mouse_highlight): If either of
 previous/next-single-property-change returns nil, treat that as
 the beginning or the end of the buffer.

src/ChangeLog

+2011-11-04  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (note_mouse_highlight): If either of
+	previous/next-single-property-change returns nil, treat that as
+	the beginning or the end of the buffer.  (Bug#9955)
+
 2011-11-04  Jan Djärv  <jan.h.d@swipnet.se>
 
-	* gtkutil.c (xg_make_tool_item): Add callbacks of one of wimage or
+	* gtkutil.c (xg_make_tool_item): Add callbacks if one of wimage or
 	label is not null (Bug#9951).
 	(xg_tool_item_stale_p): Handle the fact that wimage and/or wlbl
 	may be NULL.

src/xdisp.c

@@ -27152,8 +27152,12 @@ note_mouse_highlight (struct frame *f, int x, int y)
 		    }
 		  mouse_face_from_buffer_pos (window, hlinfo, pos,
-					      XFASTINT (before),
-					      XFASTINT (after),
+					      NILP (before)
+					      ? 1
+					      : XFASTINT (before),
+					      NILP (after)
+					      ? BUF_Z (XBUFFER (buffer))
+					      : XFASTINT (after),
 					      before_string, after_string,
 					      disp_string);
 		  cursor = No_Cursor;