Fix index out of range during shell completion

validate number of parameters to prevent panic
see https://github.com/spf13/cobra/blob/master/shell_completions.md

cmd/signaldctl/cmd/completion.go

@@ -33,15 +33,15 @@ Bash:
 	$ source <(signaldctl completion bash)
 
 	# To load completions for each session, execute once:
-Linux:
+	# Linux:
 	$ signaldctl completion bash > /etc/bash_completion.d/signaldctl
-MacOS:
+	# macOS:
 	$ signaldctl completion bash > /usr/local/etc/bash_completion.d/signaldctl
 
 Zsh:
 
-# If shell completion is not already enabled in your environment you will need
+	# If shell completion is not already enabled in your environment,
-# to enable it.  You can execute the following once:
+	# you will need to enable it.  You can execute the following once:
 
 	$ echo "autoload -U compinit; compinit" >> ~/.zshrc
 
@@ -50,30 +50,35 @@ $ signaldctl completion zsh > "${fpath[1]}/_signaldctl"
 
 	# You will need to start a new shell for this setup to take effect.
 
-Fish:
+fish:
 
 	$ signaldctl completion fish | source
 
 	# To load completions for each session, execute once:
 	$ signaldctl completion fish > ~/.config/fish/completions/signaldctl.fish
 
-Powershell:
+PowerShell:
 
 	PS> signaldctl completion powershell | Out-String | Invoke-Expression
 
 	# To load completions for every new session, run:
 	PS> signaldctl completion powershell > signaldctl.ps1
-# and source this file from your powershell profile.
+	# and source this file from your PowerShell profile.
 `,
 	DisableFlagsInUseLine: true,
 	Annotations:           map[string]string{common.AnnotationNoSocketConnection: "true"},
-	ValidArgs:             []string{"bash", "zsh"},
+	ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
+	Args:                  cobra.ExactValidArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
 		switch args[0] {
 		case "bash":
 			common.Must(cmd.Root().GenBashCompletion(os.Stdout))
 		case "zsh":
 			common.Must(cmd.Root().GenZshCompletion(os.Stdout))
+		case "fish":
+			common.Must(cmd.Root().GenFishCompletion(os.Stdout, true))
+		case "powershell":
+			common.Must(cmd.Root().GenPowerShellCompletion(os.Stdout))
 		}
 	},
 }