i#1729 offline traces: fix module tracking library exit
i#1729 offline traces: fix module tracking library exit

Adds a missing decrement of the repeat-guard in the modtrack library exit
routine.  Without it, forthcoming re-attach (i#2157) tests fail.

Review-URL: https://codereview.appspot.com/316250043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio