Fix segfault when trying to display char > 127 on a DBUS_BYTE type.
Fix segfault when trying to display char > 127 on a DBUS_BYTE type.

a > 127 char was converted to a negative number, which was registered
as a char, however ocaml char need to be in the range 0-255 otherwise
accessing the character table array would lead to negative indexing.

issue-found-by: Tomasz Wroblewski

vincenthz/ocaml-dbus