cron.ParseStandard("TZ=0") causes panic

Description

Calling cron.ParseStandard("TZ=0") causes a panic instead of returning an error.

Steps to Reproduce

package main
import "github.com/robfig/cron/v3"

func main() {
    cron.ParseStandard("TZ=0")  // Panics
}

Problem

This is problematic in production environments where:

• Users provide cron expressions as input
• A panic can crash the entire service
• Fuzz testing scenarios can easily trigger this panic

Expected Behavior

can return an error for invalid input instead of panic? like:

parser, err := cron.ParseStandard("TZ=0")
if err != nil {
    return fmt.Errorf("invalid cron expression: %w", err)
}

Environment

• Go version: 1.21+
• cron library version: latest
• OS: macOS m1