archive: Prevent assertion failure on duplicate mount
archive: Prevent assertion failure on duplicate mount

This will fix an abort when archive is to be mounted for the second time:
ERROR:gvfsbackendarchive.c:240:g_vfs_backend_archive_finalize: assertion failed: (archive->file == NULL)

https://bugzilla.redhat.com/show_bug.cgi?id=645630

philipl/gvfs,halfline/gvfs,cpatulea/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,cpatulea/gvfs,gicmo/gvfs,gicmo/gvfs,philipl/gvfs,xkahn/gvfs-cmis,philipl/gvfs,halfline/gvfs,xkahn/gvfs-cmis,xkahn/gvfs-cmis,halfline/gvfs,gicmo/gvfs