Fix crash on disconnect

The crash was due to accessing app.s.Nick() when app.s was nil.

This is a good opportunity to print a special prompt when disconnected:
I chose `<offline>`, in red.

1 files changed, 6 insertions, 0 deletions

diff --git a/app.go b/app.go
index aa6a4c4..aed5249 100644
--- a/app.go
+++ b/app.go
@@ -811,6 +811,12 @@ func (app *App) updatePrompt() {
 				StyleDefault.
 				Foreground(tcell.Color(app.cfg.Colors.Prompt)),
 		)
+	} else if app.s == nil {
+		prompt = ui.Styled("<offline>",
+			tcell.
+				StyleDefault.
+				Foreground(tcell.ColorRed),
+		)
 	} else {
 		prompt = identString(app.s.Nick())
 	}