tpool::post hangs if the maximum number of workers has already been reached, but they have all exited already

tpool_threxit_bug2.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.
# this will be unable to process more than 5 events because all of the workers have gone away.
for {set i 0} {$i < 10} {incr i} {
	tpool::post $mypool "puts \"hello $i\"; thread::exit"
}

tpool::release $mypool

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