queue stream mode change can crash ECLiPSe (0927)

Queues can be changed from read/write to update mode:

[eclipse 1]: open(queue("abc"), read, a).

yes.
[eclipse 2]: set_stream_property(a, mode, update).

yes.
[eclipse 3]: read(a,X).

X = abc
yes

If read/2 is called before the mode change, then an error is raised as
expected. However, if the mode is then changed to update, and read called
again, ECLiPSe (both main and patch branches) crashes:

[eclipse 1]: open(queue("abc"), write, a).

yes.
[eclipse 2]: read(a,X).
illegal stream mode in read_(a, X, eclipse)
[eclipse 3]: set_stream_property(a, mode, update).

yes.
[eclipse 4]: 2.
read(a, X).
Segmentation violation - possible reasons are:
- a faulty external C function
- certain operations on circular terms