[package] uci: use a delete-safe iterator in the Lua :foreach() function

The following test case exposed a NULL pointer dereference in uci_lua_foreach():
  lua -luci -e 'x=uci.cursor(); x:foreach("firewall", "forwarding", function(s) x:delete("firewall", s[".name"]) end)'

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@23757 3c298f89-4303-0410-b956-a3cf2f4a3e73