in an unintended call to write ZERO bytes to any descriptor, that was
opened for writing, when closing it.
When the descriptor belongs to a pipe, with its other end closed -- such
as when the started process has exited, doing so results in a SIGPIPE.
Tcl's core ignores SIGPIPEs, but TclX' self-tests didn't and began to
fail.
The bug should now be fixed in Tcl, but this patch is needed for TclX to
pass its own tests even when using the uncorrected version of Tcl.
