types: skip malformed derp.urls entries instead of panicking

A failed url.Parse left a nil pointer that was dereferenced at startup.

hscontrol/types/config.go

@@ -712,8 +712,8 @@ func derpConfig() DERPConfig {
 
 	urlStrs := viper.GetStringSlice("derp.urls")
 
-	urls := make([]url.URL, len(urlStrs))
-	for index, urlStr := range urlStrs {
+	urls := make([]url.URL, 0, len(urlStrs))
+	for _, urlStr := range urlStrs {
 		urlAddr, err := url.Parse(urlStr)
 		if err != nil {
 			log.Error().
@@ -721,9 +721,11 @@ func derpConfig() DERPConfig {
 				Str("url", urlStr).
 				Err(err).
 				Msg("Failed to parse url, ignoring...")
+
+			continue
 		}
 
-		urls[index] = *urlAddr
+		urls = append(urls, *urlAddr)
 	}
 
 	paths := viper.GetStringSlice("derp.paths")

hscontrol/types/derp_config_test.go

@@ -0,0 +1,28 @@
+package types
+
+import (
+	"testing"
+
+	"github.com/spf13/viper"
+)
+
+// TestDerpConfigSkipsMalformedURL ensures a malformed derp.urls entry is
+// skipped (as the "ignoring..." log promises) rather than dereferencing the
+// nil *url.URL that url.Parse returns on error, which crashed the server at
+// startup.
+func TestDerpConfigSkipsMalformedURL(t *testing.T) {
+	viper.Reset()
+	defer viper.Reset()
+
+	viper.Set("derp.urls", []string{
+		"https://controlplane.tailscale.com/derpmap/default",
+		"://bad",
+	})
+
+	cfg := derpConfig()
+
+	if len(cfg.URLs) != 1 {
+		t.Fatalf("expected the malformed derp.urls entry to be skipped, got %d urls: %v",
+			len(cfg.URLs), cfg.URLs)
+	}
+}