uci: reset uci_ptr flags when merging set operations

uci: reset uci_ptr flags when merging set operations

In some cases, e.g. when subsequently setting multiple empty option
values, uci_set() might free the section pointer of the given reused
uci_ptr structure without zeroing it, leading to a use-after-free on
processing subsequent options.

Avoid this issue by clearing the lookup pointer flags in order to
prevent uci_set() from incorrectly branching into a uci_delete()
operation leading to the freeing of the section member.

Ref: http://lists.infradead.org/pipermail/openwrt-devel/2019-October/019592.html
Reported-by: Daniel Danzberger <daniel@dd-wrt.com>
Suggested-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>