sd-network: avoid false positive compiler warning caused by LTO

Djalal Harouni <tixxdz@opendz.org>:
There is also this one genrated by LTO, IMO it's a false positive since
we do *check* for "lease" but the code is not consistent since in that
code path, "lease" is initialized to NULL in other places, except for
this one:

src/resolve/resolved-manager.c: In function 'manager_update_resolv_conf':
src/libsystemd-network/sd-dhcp-lease.c:67:18: warning: 'lease' may be used uninitialized in this function [-Wmaybe-uninitialized]
        if (lease->dns_size) {
                 ^
src/network/sd-network.c:146:24: note: 'lease' was declared here
        sd_dhcp_lease *lease;
                       ^