Fix crash after consecutive failed downloads

A segmentation fault was caused by an uninitialized mutex if an IO error
occurred on two consecutive downloads. The mutex is now explicitly
initialized on all code paths.

Also, filled out the rest of the struct just to be safe for the future.

Author: ejv2

data/cache.go

@@ -163,11 +163,15 @@ func (c *Cache) Download(item *QueueItem) (id int, err error) {
 
 	if err != nil {
 		dl = Download{
+			mut:       new(sync.RWMutex),
+			Path:      item.Path,
+			File:      f,
+			Elem:      item,
 			Started:   time.Now(),
 			Completed: true,
 			Success:   false,
-			Elem:      item,
 			Error:     "IO Error",
+			Stop: nil,
 		}
 
 		c.downloadsMutex.Lock()