Ignore invalid dates (Fixes #894)

youtube_dl/FileDownloader.py

@@ -322,6 +322,9 @@ class FileDownloader(object):
         filetime = timeconvert(timestr)
         if filetime is None:
             return filetime
+        # Ignore obviously invalid dates
+        if filetime == 0:
+            return
         try:
             os.utime(filename, (time.time(), filetime))
         except: