app/compose: Fix g_propagate_error ownership

We weren't actually transferring ownership of the error, which meant
that the error message was freed before we could even print it to the
user.

Closes: #1486
Approved by: cgwalters

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

@@ -1066,7 +1066,7 @@ impl_install_tree (RpmOstreeTreeComposeContext *self,
           }
         else
           {
-            g_propagate_error (error, temp_error);
+            g_propagate_error (error, g_steal_pointer (&temp_error));
             return FALSE;
           }
       }