xbps_sort_pkg_deps: change an assertion into a real check.
xbps_sort_pkg_deps: change an assertion into a real check.

stpx/xbps,ebfe/xbps,stpx/xbps,stpx/xbps,ebfe/xbps,datenwolf/xbps,datenwolf/xbps,ebfe/xbps,datenwolf/xbps