`{a,}{a,}a` doesn't match `a`

The following program prints `false`:

```go
package main

import (
	"fmt"
	"github.com/gobwas/glob"
)

func main() {
	g, err := glob.Compile("{a,}{a,}a")
	if err != nil {
		panic(err)
	}
	fmt.Println(g.Match("a"))
}
```