1.0.12.34: fix bug, add error signalling in RUN-PROGRAM
* Incorrect initialization of one bookkeeping variable in
COPY-DESCRIPTOR-TO-STREAM left another bookkeeping variable with
garbage after a properly-encoded short read, causing subsequent
decoding attempts to fail. Only easily visible on when the child
line buffers to one or another descriptor.
* Add test for same.
* Additionally, signal an error if we have any not-yet-decoded octets
when the child closes its output or error.
* Add test for same.