Do not map 0xFFFF in the cmap table

From the TrueType font specification section about the cmap table:

> No character code should be mapped to glyph index -1 (0xFFFF), which is a special value reserved in processing to indicate the position of a glyph deleted from the glyph stream.

ref #123

src/FontLib/Table/Type/cmap.php

@@ -196,7 +196,7 @@ function _encode() {
       $prevGid  = $gid;
     }
 
-    $segments[][] = array(0xFFFF, 0xFFFF);
+    $segments[][] = array(0xFFFF, null);
 
     $startCode = array();
     $endCode   = array();