udhcpc: shrink

udhcpc: shrink
udhcpc: guard against zero lease time

function                                             old     new   delta
timeout                                                4       -      -4
server_addr                                            4       -      -4
requested_ip                                           4       -      -4
perform_release                                      134     112     -22
udhcpc_main                                         2511    2485     -26
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(add/remove: 0/3 grow/shrink: 0/2 up/down: 0/-60)             Total: -60 bytes