[ASAN] metalink: Fix leaks of buffer
[ASAN] metalink: Fix leaks of buffer

We should be religious about the "only set output variables on
success", otherwise it makes leaks more likely.

But the real leak was us simply not using autoptr in one place.

Closes: #598
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree