uci: fix options list of section after type change

uci: fix options list of section after type change

A section can store its name in the same memory region as the section
(after the actual section object). The object is then reallocated when the
type is later changed via an uci_set. But the original address of the
section is (indirectly) stored in the section list, the object and the
object list (HEAD) of this section.

But only the section list was fixed in commit 4fb6a564b8ee ("clean up
uci_set") after the realloc finished. Traversing the object list or
accessing the section pointer caused heap-use-after-free errors.

Reported-by: Charlemagne Lasse <charlemagnelasse@gmail.com>
Fixes: 4fb6a564b8ee ("clean up uci_set")
Signed-off-by: Sven Eckelmann <sven@narfation.org>