tpool::release hangs if any of the thread pool's workers have exited

tpool_threxit_bug1.tcl

#!/usr/local/bin/tclsh

package require Thread

# initialize the thread pool
set mypool [tpool::create -minworkers 1 -maxworkers 5]

# queue up something that exits the thread
tpool::post $mypool "puts \"hello\"; thread::exit"

puts "releasing pool (this hangs)"
tpool::release $mypool

This is because TpoolRelease blindly signals and waits for the workers to exit, without checking if they are actually still running: https://github.com/tcltk/thread/blob/master/generic/threadPoolCmd.c#L1650

admittedly, using thread::exit within a tpool is not a normal or recommended activity, but maybe this should be handled and detected better.

Reported at http://core.tcl.tk/thread/tktview/646509f34f5e258709009f46ba26206969154f7f