cli: Fix panic on empty data argument to kv put

Passing in an empty quoted argument from the shell currently panics as
we never check the length being greater than 0 prior to indexing into
the first rune, as illustrated in the test in this commit.

We also fix the panic, treating an empty string for data as equivalent
to not having passed it in the first place.

command/kv_put.go

@@ -236,6 +236,11 @@ func (c *KVPutCommand) dataFromArgs(args []string) (string, string, error) {
 	key := args[0]
 	data := args[1]
 
+	// Handle empty quoted shell parameters
+	if len(data) == 0 {
+		return key, "", nil
+	}
+
 	switch data[0] {
 	case '@':
 		data, err := ioutil.ReadFile(data[1:])

command/kv_put_test.go

@@ -101,6 +101,34 @@ func TestKVPutCommand_Run(t *testing.T) {
 	}
 }
 
+func TestKVPutCommand_RunEmptyDataQuoted(t *testing.T) {
+	srv, client := testAgentWithAPIClient(t)
+	defer srv.Shutdown()
+	waitForLeader(t, srv.httpAddr)
+
+	ui := new(cli.MockUi)
+	c := &KVPutCommand{Ui: ui}
+
+	args := []string{
+		"-http-addr=" + srv.httpAddr,
+		"foo", "",
+	}
+
+	code := c.Run(args)
+	if code != 0 {
+		t.Fatalf("bad: %d. %#v", code, ui.ErrorWriter.String())
+	}
+
+	data, _, err := client.KV().Get("foo", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if data.Value != nil {
+		t.Errorf("bad: %#v", data.Value)
+	}
+}
+
 func TestKVPutCommand_RunBase64(t *testing.T) {
 	srv, client := testAgentWithAPIClient(t)
 	defer srv.Shutdown()