Do not segfault on sending to closed socket

carlin++ for reporting the error.
The problem was that send() and write() tested $!PIO for truthness,
but a closed socket does not evaluate to False on JVM and Moar

src/core/IO/Socket.pm

@@ -146,6 +146,7 @@ my role IO::Socket does IO {
 #?endif
 #?if !parrot
         nqp::closefh($!PIO);
+        $!PIO := Mu;
         True
 #?endif
     }