lwip: dhcp-server fix static analysis warnings
lwip: dhcp-server fix static analysis warnings

1) kill_oldest_dhcps_pool() is only called when list has at least two members (assured with kconfig value limit), added assertion to ensure this function is used only when prerequisities are met
2) use after free reported in two places, since the analyzer checks also the scenario when the linked list has loops, added ignore tags

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf