[CFF] Use NNOffsetTo<> instead of OffsetTo<>

I'm pretty sure that's what is intended.
diff --git a/src/hb-ot-cff1-table.hh b/src/hb-ot-cff1-table.hh
index 7228f77..157483b 100644
--- a/src/hb-ot-cff1-table.hh
+++ b/src/hb-ot-cff1-table.hh
@@ -1390,7 +1390,7 @@
 
   public:
   FixedVersion<HBUINT8> version;	  /* Version of CFF table. set to 0x0100u */
-  OffsetTo<CFF1NameIndex, HBUINT8> nameIndex; /* headerSize = Offset to Name INDEX. */
+  NNOffsetTo<CFF1NameIndex, HBUINT8> nameIndex; /* headerSize = Offset to Name INDEX. */
   HBUINT8	       offSize;	  /* offset size (unused?) */
 
   public: