libpriv/util: add precondition to catch potential trap

Basically, it doesn't make sense for the caller to only want the
pkglist, but not the refsack because the former has a more limited
lifetime than the latter. Check for that to make sure nobody falls in
this trap like I did.

Closes: #1134
Approved by: cgwalters

src/libpriv/rpmostree-rpm-util.c

@@ -890,6 +890,10 @@ rpmostree_get_pkglist_for_root (int               dfd,
                                 GCancellable     *cancellable,
                                 GError          **error)
 {
+  /* the DnfPackage objects don't have a ref on their DnfSack; make sure callers don't fall
+   * in that trap */
+  g_return_val_if_fail (out_pkglist == NULL || out_refsack != NULL, FALSE);
+
   g_autoptr(RpmOstreeRefSack) refsack =
     rpmostree_get_refsack_for_root (dfd, path, cancellable, error);
   if (!refsack)