Initialize variable ifr in sk_setup()

Initialize variable ifr in sk_setup()

==00:00:00:02.831 2468== Syscall param socketcall.setsockopt(optval) points to uninitialised byte(s)
==00:00:00:02.831 2468==    at 0x513BDEA: setsockopt (in /usr/lib/libc-2.23.so)
==00:00:00:02.831 2468==    by 0x45C7AF: sk_setup (io.c:1216)
==00:00:00:02.831 2468==    by 0x45CDFF: sk_open (io.c:1417)
==00:00:00:02.831 2468==    by 0x44B562: rip_open_socket (packets.c:740)
==00:00:00:02.831 2468==    by 0x4481A7: rip_iface_locked (rip.c:616)
==00:00:00:02.831 2468==    by 0x4133E4: olock_run_event (locks.c:177)
==00:00:00:02.831 2468==    by 0x45A6DE: ev_run (event.c:85)
==00:00:00:02.831 2468==    by 0x45A7AD: ev_run_list (event.c:142)
==00:00:00:02.831 2468==    by 0x45E0FC: io_loop (io.c:2066)
==00:00:00:02.831 2468==    by 0x463B56: main (main.c:845)
==00:00:00:02.831 2468==  Address 0xffefffd24 is on thread 1's stack
==00:00:00:02.831 2468==  in frame #1, created by sk_setup (io.c:1188)
==00:00:00:02.831 2468==  Uninitialised value was created by a stack allocation
==00:00:00:02.831 2468==    at 0x45C6BB: sk_setup (io.c:1188)