read_exdr/2 hangs on output streams (#0896)

According to the bips page, read_exdr/2 should behave like read/2, so
that when the stream is not an input stream, error 192 (illegal stream
mode) is raised:

[eclipse 15]: read(output, X).
illegal stream mode in read_(output, X, eclipse)

but read_exdr on an output stream (I tried output
and debug_output) does not return:

[eclipse 17]: read_exdr(output, X).
^C

interruption: type a, b, c, e, or h for help : ? abort
Aborting execution ...
[eclipse 18]: read_exdr(debug_output, X).
^C

interruption: type a, b, c, e, or h for help : ? continue
^C

interruption: type a, b, c, e, or h for help : ? abort
Aborting execution ...
[eclipse 19]: 

(write_exdr/2 on an input stream does raise 192:

[eclipse 19]: write_exdr(input, a).
illegal stream mode in write_exdr(input, a)
[eclipse 20]:
)