uci: fix memory leak in rpc_uci_replace_savedir()

uci: fix memory leak in rpc_uci_replace_savedir()

The rpc_uci_replace_savedir() function did not take into account that libuci
uci_set_savedir() does an additional implicit uci_strdup() of the directory
path string when appending a new delta directory item.

Due to this oversight, only the struct uci_element items got freed, but not
the duplicated path string, leading to leaking memory when invoking the uci
api with session id argument.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>