m/n/core: fix crash on exit

The current code crashes on exit if a console wasn't opened.

Change-Id: I7fc235953fe04b8ea924a380e7eb9195a8883d6d
Reviewed-on: https://review.monogon.dev/c/monogon/+/1387
Reviewed-by: Leopold Schabel <leo@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/node/core/main.go b/metropolis/node/core/main.go
index 3c77e92..680073e 100644
--- a/metropolis/node/core/main.go
+++ b/metropolis/node/core/main.go
@@ -205,6 +205,9 @@
 	time.Sleep(time.Second)
 	// After a bit, kill all console log readers.
 	for _, console := range consoles {
+		if console.reader == nil {
+			continue
+		}
 		console.reader.Close()
 		console.reader.Stream = nil
 	}