revert parse_xrefs to its original signature

Passing the aux struct by reference may look cleaner, but it was
deliberate to keep parse_xrefs() independent of that struct, since the
latter is conceptually part of the parser's interface and the former is
not.

Also, this way parse_xrefs() has a proper return value that signals
success or failure.

Plus, no ugly indirection or temporary variable is needed to access sz.