cli: fix bug that caused help to panic (#11157)

vlib/cli/help_test.v

@@ -0,0 +1,65 @@
+module cli
+
+fn test_help_message() {
+	mut cmd := Command{
+		name: 'command'
+		description: 'description'
+		commands: [
+			Command{
+				name: 'sub'
+				description: 'subcommand'
+			},
+			Command{
+				name: 'sub2'
+				description: 'another subcommand'
+			},
+		]
+		flags: [
+			Flag{
+				flag: .string
+				name: 'str'
+				description: 'str flag'
+			},
+			Flag{
+				flag: .bool
+				name: 'bool'
+				description: 'bool flag'
+				abbrev: 'b'
+			},
+			Flag{
+				flag: .string
+				name: 'required'
+				abbrev: 'r'
+				required: true
+			},
+		]
+	}
+	assert cmd.help_message() == r'Usage: command [flags] [commands]
+
+description
+
+Flags:
+      -str            str flag
+  -b  -bool           bool flag
+  -r  -required       (required)
+
+Commands:
+  sub                 subcommand
+  sub2                another subcommand
+'
+
+	cmd.posix_mode = true
+	assert cmd.help_message() == r'Usage: command [flags] [commands]
+
+description
+
+Flags:
+      --str           str flag
+  -b  --bool          bool flag
+  -r  --required      (required)
+
+Commands:
+  sub                 subcommand
+  sub2                another subcommand
+'
+}