pull: Fix crash specifying override URL in summary fetch
pull: Fix crash specifying override URL in summary fetch

The summary URL override is looked up with "&s", which directly
exchanges the data to a pointer without allocation. This was causing a
segfault calling ostree_repo_remote_fetch_summary_with_options from
pygobject.

Closes: #829
Approved by: jlebon

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