[subset] Fix cast-align issue

Fixes https://github.com/harfbuzz/harfbuzz/issues/4316

src/hb-serialize.hh

@@ -266,7 +266,7 @@ struct hb_serialize_context_t
 	   propagate_error (std::forward<Ts> (os)...); }
 
   /* To be called around main operation. */
-  template <typename Type>
+  template <typename Type=char>
   __attribute__((returns_nonnull))
   Type *start_serialize ()
   {

src/hb-subset.cc

@@ -273,7 +273,7 @@ _try_subset (const TableType *table,
              hb_vector_t<char>* buf,
              hb_subset_context_t* c /* OUT */)
 {
-  c->serializer->start_serialize<TableType> ();
+  c->serializer->start_serialize ();
   if (c->serializer->in_error ()) return false;
 
   bool needed = table->subset (c);