uci: fix a potential use-after-free in uci_set()

uci: fix a potential use-after-free in uci_set()

When calling uci_set() to update an option, if ptr->o != NULL and
ptr->option == NULL, then uci_expand_ptr() will set ptr->option to
ptr->o->e.name (or the caller could set ptr->option to that value). In
this case, the option will be freed just before calling
uci_alloc_option() with ptr->option, which was just freed.

Signed-off-by: Jordan Miner <jminer7@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>