compose: Clear libdnf context before postprocessing

This ensures we close the rpm database before we start moving
things around during postprocessing, and avoids a warning about
the rpmdb going missing being printed at the end of a compose.

Related: https://github.com/projectatomic/rpm-ostree/issues/987

Closes: #989
Approved by: jlebon

src/app/rpmostree-compose-builtin-tree.c

@@ -915,6 +915,11 @@ impl_compose_tree (const char      *treefile_pathstr,
       }
   }
 
+  /* Destroy this now so the libdnf stack won't have any references
+   * into the filesystem before we manipulate it.
+   */
+  g_clear_object (&self->corectx);
+
   if (g_strcmp0 (g_getenv ("RPM_OSTREE_BREAK"), "post-yum") == 0)
     return FALSE;