Remove null characters from path names

A user reported that their manually-entered genre tags had null
characters at the end, after reloading them into Picard later. It's not
clear if Picard somehow added these, or another program did.
Regardless, using %genre% in the file naming scheme caused saving to
fail. Null characters are now removed from path names before saving.

picard/file.py

@@ -227,6 +227,8 @@ def _script_to_filename(self, format, file_metadata, settings):
             if isinstance(filename, unicode):
                 filename = unaccent(filename)
             filename = replace_non_ascii(filename)
+        # remove null characters
+        filename = filename.replace("\x00", "")
         return filename
 
     def _make_filename(self, filename, metadata, settings):