Fix dial panic when ctx is nil

dial.go

@@ -157,7 +157,10 @@ func handshakeRequest(ctx context.Context, urls string, opts *DialOptions, copts
 return nil, fmt.Errorf("unexpected url scheme: %q", u.Scheme)
 }
 
-req, _ := http.NewRequestWithContext(ctx, "GET", u.String(), nil)
+req, err := http.NewRequestWithContext(ctx, "GET", u.String(), nil)
+if err != nil {
+return nil, fmt.Errorf("failed to build HTTP request: %w", err)
+}
 req.Header = opts.HTTPHeader.Clone()
 req.Header.Set("Connection", "Upgrade")
 req.Header.Set("Upgrade", "websocket")

dial_test.go

@@ -23,10 +23,11 @@ func TestBadDials(t *testing.T) {
 t.Parallel()
 
 testCases := []struct {
-name string
-url string
-opts *DialOptions
-rand readerFunc
+name string
+url string
+opts *DialOptions
+rand readerFunc
+nilCtx bool
 }{
 {
 name: "badURL",
@@ -46,15 +47,24 @@ func TestBadDials(t *testing.T) {
 return 0, io.EOF
 },
 },
+{
+name: "nilContext",
+url: "http://localhost",
+nilCtx: true,
+},
 }
 
 for _, tc := range testCases {
 tc := tc
 t.Run(tc.name, func(t *testing.T) {
 t.Parallel()
 
-ctx, cancel := context.WithTimeout(context.Background(), time.Second*5)
-defer cancel()
+var ctx context.Context
+var cancel func()
+if !tc.nilCtx {
+ctx, cancel = context.WithTimeout(context.Background(), time.Second*5)
+defer cancel()
+}
 
 if tc.rand == nil {
 tc.rand = rand.Reader.Read