Fixed stacktrace error

[yglukhov]

Fixes dom96/jester#65
Simpler steps to reproduce:

import locks

{.push stacktrace:on.}

proc someProc*() =
    raise newException(Exception, "")

{.pop.}

proc serve() =
    try:
        echo 1
        someProc()
    except:
        echo 2

    try:
        echo 3
        someProc()
        echo 4
    except:
        echo 5

serve()

This code hangs.