Fix use of uninitialized afhi->header_len.

In aft's add_one_audio_file() we set the header len and
the header offset to zero so that the get_file_info()
functions of the audio format handlers that do not use
headers don't need to zero these fields.

However, this is not the right place to do that as para_afh
also calls get_file_info() via compute_afhi() but missed to
clear these fields which leads to a segmentation fault.

Fix this bug by moving the zeroing to compute_afhi().