fix(storage): fix Writer.ChunkSize validation (#4255)

Since w.opened is set to true before this validation currently,
subsequent calls to Writer.Close will deadlock. There is no
reason not to do this check first before opening the pipe.

Thanks to @dt for pointing this out.

Updates #4167

storage/writer.go

@@ -99,15 +99,15 @@ func (w *Writer) open() error {
 	if attrs.KMSKeyName != "" && w.o.encryptionKey != nil {
 		return errors.New("storage: cannot use KMSKeyName with a customer-supplied encryption key")
 	}
+	if w.ChunkSize < 0 {
+		return errors.New("storage: Writer.ChunkSize must be non-negative")
+	}
 	pr, pw := io.Pipe()
 	w.pw = pw
 	w.opened = true
 
 	go w.monitorCancel()
 
-	if w.ChunkSize < 0 {
-		return errors.New("storage: Writer.ChunkSize must be non-negative")
-	}
 	mediaOpts := []googleapi.MediaOption{
 		googleapi.ChunkSize(w.ChunkSize),
 	}