fixes taken pathes in the primus promiscuous mode (#971)

We had a bug so far when pathes that were taken in
the primus promiscuous mode sometimes were far away
from the real ones.
That was possible because a forked continuation could
return and an execution would continue from a forked
point.

This PR fixes this bug and a forked continuation
never returns.

plugins/primus_promiscuous/primus_promiscuous_main.ml

@@ -146,7 +146,9 @@ module Main(Machine : Primus.Machine.S) = struct
         Machine.Global.get state >>= fun {forkpoints} ->
         if Set.mem forkpoints dst
         then Eval.halt >>= never_returns
-        else Linker.exec (`tid dst)
+        else
+          Linker.exec (`tid dst) >>= fun () ->
+          Eval.halt >>= never_returns
     | _ -> Machine.return ()
 
   let fork_on_calls blk jmp = match Jmp.kind jmp with