[ffmpeg] Correct argument encoding on Windows with Python 2.x

Fixes #2924

youtube_dl/utils.py

@@ -540,6 +540,16 @@ def encodeFilename(s, for_subprocess=False):
         encoding = 'utf-8'
     return s.encode(encoding, 'ignore')
 
+
+def encodeArgument(s):
+    if not isinstance(s, compat_str):
+        # Legacy code that uses byte strings
+        # Uncomment the following line after fixing all post processors
+        #assert False, 'Internal error: %r should be of type %r, is %r' % (s, compat_str, type(s))
+        s = s.decode('ascii')
+    return encodeFilename(s, True)
+
+
 def decodeOption(optval):
     if optval is None:
         return optval