ObjectInputStream: fix a quirk

When reading a handle, we really need to subtract the HANDLE_OFFSET
in order to get the correct number. Therefore, either the variable
name needs to be changed to "handlePlusOffset" or the offset has to
be subtracted earlier.

This developer chose the latter solution.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ObjectInputStream: superclass' descriptors can be references, too

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>