Adds verification to number of args for completion cmd

The completion cmd requires at least one extra argument but it
doesn't check cobra args before switch statement, causing panic.
This commit fixes #210.

(cherry picked from commit e5c62cc)

pkg/cmd/completion/completion.go

@@ -47,11 +47,13 @@ func Command() *cobra.Command {
 		ValidArgs: []string{"bash", "zsh"},
 		Example:   eg,
 		RunE: func(cmd *cobra.Command, args []string) error {
-			switch args[0] {
-			case "bash":
-				return cmd.Root().GenBashCompletion(os.Stdout)
-			case "zsh":
-				return runCompletionZsh(os.Stdout, cmd.Parent())
+			if len(args) == 1 {
+				switch args[0] {
+				case "bash":
+					return cmd.Root().GenBashCompletion(os.Stdout)
+				case "zsh":
+					return runCompletionZsh(os.Stdout, cmd.Parent())
+				}
 			}
 			return nil
 		},