Fix build with -O0

message_impl was not defined.  That causes trouble if compiler didn't
optimize the unreachable call out...

src/hb-buffer.cc

@@ -2023,7 +2023,6 @@ hb_buffer_set_message_func (hb_buffer_t *buffer,
     buffer->message_destroy = nullptr;
   }
 }
-
 bool
 hb_buffer_t::message_impl (hb_font_t *font, const char *fmt, va_list ap)
 {

src/hb-buffer.hh

@@ -359,6 +359,9 @@ struct hb_buffer_t
   }
   bool message (hb_font_t *font, const char *fmt, ...) HB_PRINTF_FUNC(3, 4)
   {
+#ifdef HB_NO_BUFFER_MESSAGE
+   return true;
+#else
     if (!messaging ())
       return true;
     va_list ap;
@@ -366,6 +369,7 @@ struct hb_buffer_t
     bool ret = message_impl (font, fmt, ap);
     va_end (ap);
     return ret;
+#endif
   }
   HB_INTERNAL bool message_impl (hb_font_t *font, const char *fmt, va_list ap) HB_PRINTF_FUNC(3, 0);