Free the old XML strings before overwriting them if the user
has chosen to reedit the file or force the redefinition.
Found by Alex Jia trying to reproduce another bug:
https://bugzilla.redhat.com/show_bug.cgi?id=977430#c3
(cherry picked from commit 1e3a252974c8e5c650f1d84dc2b167f0ae8cee3c)