fix(validity): skip put_data on zero-length 'd' format in pack

UBSan flags null pointer passed to memcpy when validity_pack_new
is called with 'd' format, NULL data, and length 0 (e.g.
cmd_get_user_storage with NULL name).

libfprint/drivers/validity/validity_pack.h

@@ -63,6 +63,8 @@ validity_pack_one (FpiByteWriter *w,
       {
         const guint8 *d = va_arg (*ap, const guint8 *);
         gsize len = va_arg (*ap, gsize);
+        if (len == 0)
+          return TRUE;
         return fpi_byte_writer_put_data (w, d, len);
       }