Disable station mode selection for QuickMix

It does not have any modes anyway.

Fixes #723.

src/ui_act.c

@@ -814,15 +814,22 @@ BarUiActCallback(BarUiActManageStation) {
 		strcat (question, "[f]eedback");
 		*allowedPos++ = 'f';
 	}
-	/* station mode is always available */
 	if (allowedPos != allowedActions) {
 		strcat (question, "? ");
 	}
-	strcat (question, "Manage [m]ode? ");
-	*allowedPos++ = 'm';
+	/* station mode is not available for QuickMix. */
+	if (!selStation->isQuickMix) {
+		strcat (question, "Manage [m]ode? ");
+		*allowedPos++ = 'm';
+	}
 
 	*allowedPos = '\0';
 
+	if (allowedPos == allowedActions) {
+		BarUiMsg (&app->settings, MSG_INFO, "No actions available.\n");
+		return;
+	}
+
 	assert (strlen (question) < sizeof (question) / sizeof (*question));
 
 	BarUiMsg (&app->settings, MSG_QUESTION, "%s", question);