This is easily triggered via a `pacman -Sc` operation when it attempts
to open a delta file as a package- we end up leaking loads of memory
due to us never freeing the archive object. When you have upwards of
1200 delta files in your sync database directory, this results in a
memory leak of nearly 1.5 MiB.
Also fix another memory leak noticed at the same time- we need to call
the internal _alpm_pkg_free() function, as without the origin data being
set the public free function will do nothing.