Fix [rt.cpan.org #93607] refcount broken in undocumented functions

Reported by Zefram. https://rt.cpan.org/Ticket/Display.html?id=93607

Report:
The undocumented decode_with_header{,_and_offset} functions screw
up the refcount of the elements being put into the returned array.
av_store takes ownership of one counted reference, but the code doesn't
increment the refcount for that purpose.

Fix:
Added SvREFCNT_inc() to the relevant calls. Thanks for the report.