net: disallow dialing and listening on port > 65535 on Plan 9

Since Dial(":80") was implemented on Plan 9 (CL 32593),
TestProtocolDialError is failing because dialing a port
superior to 65535 is supported on Plan 9.

This change disallows dialing and listening on ports
superior to 65535.

Fixes #17761.

Change-Id: I95e8a163eeacf1ccd8ece7b650f16a0531c59709
Reviewed-on: https://go-review.googlesource.com/32594Reviewed-by: Russ Cox <rsc@golang.org>

src/net/ipsock_plan9.go

@@ -119,6 +119,11 @@ func startPlan9(ctx context.Context, net string, addr Addr) (ctl *os.File, dest,
 		return
 	}
 
+	if port > 65535 {
+		err = InvalidAddrError("port should be < 65536")
+		return
+	}
+
 	clone, dest, err := queryCS1(ctx, proto, ip, port)
 	if err != nil {
 		return