Check for standalone device IDs as args for `paths` command

Change-Id: Ia821345bbee2a9910d64fa37e033dec60d0da869
diff --git a/cli/src/main/java/org/onosproject/cli/net/PathListCommand.java b/cli/src/main/java/org/onosproject/cli/net/PathListCommand.java
index 141ae26..5bdd92b 100644
--- a/cli/src/main/java/org/onosproject/cli/net/PathListCommand.java
+++ b/cli/src/main/java/org/onosproject/cli/net/PathListCommand.java
@@ -50,6 +50,10 @@
     @Override
     protected void execute() {
         init();
+        if (src.split("/").length != 1 || dst.split("/").length != 1) {
+            print("Expected device IDs as arguments");
+            return;
+        }
         Set<Path> paths = service.getPaths(topology, deviceId(src), deviceId(dst));
         if (outputJson()) {
             print("%s", json(this, paths));