close socket explicitly (idempotent);

--- a/src/Pure/System/isabelle_process.ML	Tue Mar 31 14:09:36 2020 +0200
+++ b/src/Pure/System/isabelle_process.ML	Tue Mar 31 14:40:56 2020 +0200
@@ -204,6 +204,8 @@
     val _ = Future.shutdown ();
     val _ = Execution.reset ();
     val _ = Message_Channel.shutdown msg_channel;
+    val _ = BinIO.closeIn in_stream;
+    val _ = BinIO.closeOut out_stream;
 
   in Exn.release result end);