libotutil: Avoid infinite recursion during error unwinding

libotutil: Avoid infinite recursion during error unwinding

When we clean up from an error, for example copy_file_range() failing
while we generate a static delta (perhaps caused by
https://gitlab.gnome.org/GNOME/libglnx/-/issues/3 or by a
genuine write error), we might free a variant builder that has a
non-null parent. Previously, this caused infinite recursion and a stack
overflow, repeatedly freeing the same object, but Luca Bruno suggested
that the intention here appears to have been to free the parent object.

Partially resolves https://github.com/ostreedev/ostree/issues/2525
(the other bug reported in that issue needs to be resolved by updating
libglnx to a version where libglnx#3 has been fixed).

Signed-off-by: Simon McVittie <smcv@collabora.com>