This changes the signature of _alpm_pkg_dup() to return an integer error
code and provide the new package in a passed pointer argument. All
callers are now more robust with checking the return value of this
function to ensure a fatal error did not occur.
We allow load failures to proceed as otherwise we have a chicken and egg
problem- if a 'desc' local database entry is missing, the best way of
restoring said file is `pacman -Sf --dbonly packagename`. This patch
fixes a segfault that was occurring in this case.