Use NoSymbol as the error code

This is how the X11 API does things, so it's clearer if we use
the same principle.
diff --git a/vncviewer/keysym2ucs.c b/vncviewer/keysym2ucs.c
index 4e3dca2..2fd5870 100644
--- a/vncviewer/keysym2ucs.c
+++ b/vncviewer/keysym2ucs.c
@@ -34,6 +34,8 @@
 
 #include "keysym2ucs.h"
 
+#define NoSymbol 0
+
 struct codepair {
   unsigned short keysym;
   unsigned short ucs;
@@ -886,5 +888,5 @@
     return ucs | 0x01000000;
 
   /* no matching keysym value found */
-  return 0xFFFFFF;
+  return NoSymbol;
 }