[downloader:http] treat 416 without downloaded data as error

Downloading https://pbs.twimg.com/media/EB2cGUYX4AI2Vuu.jpg:orig (NSFW)
sometimes returns a 416 status code, even though no 'Range' header was
sent and no data was downloaded prior.
This code usually means a file has already been downloaded completely
and the download method indicates success, but in this case it causes
an exception down the pipeline since no file was created.

gallery_dl/downloader/http.py

@@ -103,7 +103,7 @@ class HttpDownloader(DownloaderBase):
             elif code == 206:  # Partial Content
                 offset = filesize
                 size = response.headers["Content-Range"].rpartition("/")[2]
-            elif code == 416:  # Requested Range Not Satisfiable
+            elif code == 416 and filesize:  # Requested Range Not Satisfiable
                 break
             else:
                 msg = "{}: {} for url: {}".format(code, response.reason, url)