local-addresses: fix use of uninitialized value

local-addresses: fix use of uninitialized value

This can happen if ifi fails to be read from the netlink message and the
error is ENODATA.

Fixes the following valgrind message when running netstat:

==164141== Conditional jump or move depends on uninitialised value(s)
==164141==    at 0x524AE60: address_compare (local-addresses.c:29)
==164141==    by 0x48BCC78: msort_with_tmp.part.0 (msort.c:105)
==164141==    by 0x48BC9E4: msort_with_tmp (msort.c:45)
==164141==    by 0x48BC9E4: msort_with_tmp.part.0 (msort.c:53)
==164141==    by 0x48BCF85: msort_with_tmp (msort.c:45)
==164141==    by 0x48BCF85: qsort_r (msort.c:297)
==164141==    by 0x52500FC: UnknownInlinedFun (sort-util.h:47)
==164141==    by 0x52500FC: local_gateways.constprop.0 (local-addresses.c:310)
==164141==    by 0x5251C05: _nss_myhostname_gethostbyaddr2_r (nss-myhostname.c:456)
==164141==    by 0x5252006: _nss_myhostname_gethostbyaddr_r (nss-myhostname.c:500)
==164141==    by 0x498E7FE: gethostbyaddr_r@@GLIBC_2.2.5 (getXXbyYY_r.c:274)
==164141==    by 0x498E560: gethostbyaddr (getXXbyYY.c:135)
==164141==    by 0x121353: INET_rresolve.constprop.0 (inet.c:212)
==164141==    by 0x1135B9: INET_sprint (inet.c:261)
==164141==    by 0x121BFC: addr_do_one.constprop.0.isra.0 (netstat.c:1156)