synchronize access to the cache

the docs say that access from multiple goroutines is undefined.

go/favicon.go

	"net/http"
	"net/url"
	"os"
	"sync"
)
var faviconCache = make(map[string]string)
var lock sync.RWMutex
func main() {
	http.HandleFunc("/favicon", HandleGetFavicon)
	} else {
		host = parsed.Host
	}
	lock.RLock()
	faviconUrl, cached := faviconCache[host]
	lock.RUnlock()
	if cached {
		return faviconUrl, nil
		return faviconUrl, err
	}
	lock.Lock()
	faviconCache[host] = faviconUrl
	lock.Unlock()
	return faviconUrl, nil
}