fix bug caused by fienames starting with - (spotDL#1413)

Author: s1as3r

spotdl/download/ffmpeg.py

@@ -61,8 +61,8 @@ async def convert(
     # ! sampled length of songs matches the actual length (i.e. a 5 min song won't display
     # ! as 47 seconds long in your music player, yeah that was an issue earlier.)
 
-    downloaded_file_path = str(downloaded_file_path)
-    converted_file_path = str(converted_file_path)
+    downloaded_file_path = str(downloaded_file_path.absolute())
+    converted_file_path = str(converted_file_path.absolute())
 
     formats = {
         "mp3": ["-codec:a", "libmp3lame"],