(Fx_show_tip): Set string to " " if empty.

src/ChangeLog

+2008-02-18  Jan Dj$(Q)Z(Brv  <jan.h.d@swipnet.se>
+
+	* xfns.c (Fx_show_tip): Set string to " " if empty.
+
 2008-02-17  Dan Nicolaescu  <dann@ics.uci.edu>
 
 	* callint.c (syms_of_callint): Initialize Vmark_even_if_inactive

src/xfns.c

@@ -5317,6 +5317,9 @@ Text larger than the specified size is clipped.  */)
   GCPRO4 (string, parms, frame, timeout);
 
   CHECK_STRING (string);
+  if (SCHARS (string) == 0)
+    string = make_unibyte_string (" ", 1);
+
   f = check_x_frame (frame);
   if (NILP (timeout))
     timeout = make_number (5);