vrrp: fix segfault when instance has no interface configured #567

Job	Run time
build	1m 2s
build (--enable-snmp --enable-snmp-rfc --enable-json --enable-dbus --disable-checksum-compat --en...	55s
build (--enable-dbus --enable-dbus-create-instance --disable-dynamic-linking --disable-fwmark --d...	56s
build (--enable-dynamic-linking --disable-vrrp-auth --enable-snmp-rfc --disable-snmp-reply-v3-for...	49s
build (--disable-libnl --enable-snmp-checker --disable-vmac)	52s
build (--enable-conversion-checks --enable-stacktrace --enable-mem-check --enable-mem-check-log -...	55s
build (--disable-lvs --enable-snmp-vrrp --enable-snmp-rfc --enable-json --enable-dbus --disable-r...	46s
build (--disable-vrrp --enable-snmp-checker --enable-regex)	44s
build (--disable-hardening --enable-dump-threads --enable-epoll-debug --enable-snmp-rfcv3 --enabl...	50s
build (--enable-snmp-rfc --enable-snmp --enable-dbus --enable-json --enable-bfd --enable-regex --...	56s
	8m 45s

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