[ns] Strange bug in stable ns

[Alex Newman]

So in the most recent version of ns on freebsd I got a strange error on one
out of 100 runs of the sim. 


ns: _o1101 recv 55 42 1400 {ack:307:50}: can't read "acks(307)": no such element
 in array
    while executing
"if { [info exists acks($msg_id)] > 0 && $acks($msg_id) > 0 } {
                if { $destination == [$node_ node-addr]} {
                        if { $val(debug_ack) == "on" } {
                                $n..."
    (procedure "_o1101" line 15)
    (Agent/MessagePassing/Flooding ack_rcv line 15)
    invoked from within
"$self ack_rcv $source $sport $size $data"
    (procedure "_o1101" line 10)
    (Agent/MessagePassing/Flooding recv line 10)
    invoked from within
"_o1101 recv 55 42 1400 {ack:307:50}"

Note I check if that msg exists before it crashes because it doesn't exist.
Does tcl not check in linear order. That seems very strange to me.
-Alex