Minor cleanups.

Makes style slightly more consistent.
Also fixes the return code of op_fetch_headers() to make it
 consistently return OP_EBADHEADER if the stream runs out of pages
 after a valid OpusHead packet is found.
Previously, if a valid OpusHead was found, it would return
 OP_ENOTFORMAT if it ran out of pages before finding one without
 its BOS flag set, and OP_EBADHEADER if it ran out of pages after
 finding one without its BOS flag set.