pull: Ensure temporary data that appears corrupted is deleted

If a MITM attacker (or just network corruption) causes a temporary
downloaded object in tmp/ to be corrupted, we'll end up
continually trying to commit it, and fail.

Fix this unlinking the temp file immediately after opening it.  This
will ensure that if we exit due to an error (or crash), the kernel
will clean up the space for us.

https://bugzilla.gnome.org/show_bug.cgi?id=725924