cli: fix a panic and an infinite loop, when command flag descriptions have multiple lines (#17981)

vlib/cli/cli_test.v

@@ -0,0 +1,25 @@
+module main
+
+import cli { Command, Flag }
+
+fn test_long_description() {
+	mut cmd := Command{
+		name: 'cli'
+		description: 'An example of the cli library.'
+		version: '1.0.0'
+	}
+	mut greet_cmd := Command{
+		name: 'greet'
+		description: 'Prints greeting in different languages.'
+		usage: '<name>'
+		required_args: 1
+	}
+	greet_cmd.add_flag(Flag{
+		flag: .string_array
+		name: 'fun'
+		description: '\'{"uri":"mqtt://broker.emqx.io:1883","topic":"test_emq/1","filters":[{"producer_id":0,"trace_group":2,"string_id":3001},{"string_id":3002}]}\''
+	})
+	cmd.add_command(greet_cmd)
+	cmd.setup()
+	cmd.parse(['cli', 'greet', '-help'])
+}

vlib/cli/help.v

@@ -153,8 +153,8 @@ fn pretty_description(s string, indent_len int) string {
 		mut i := chars_per_line - 2
 		mut j := 0
 		for ; i < line.len; i += chars_per_line - 2 {
-			for line[i] != ` ` {
-				i--
+			for j > 0 && line[j] != ` ` {
+				j--
 			}
 			// indent was already done the first iteration
 			if j != 0 {