virsh-network: Avoid possible NULL deref in cmdNetworkDHCPLeases

virsh-network: Avoid possible NULL deref in cmdNetworkDHCPLeases

Problem is, localtime_r() returns a pointer to converted time or
NULL in case of an error. But checking the glibc sources, error
will occur iff a NULL has been passed as an either of arguments
the function takes. But GCC fails to see that:

../../tools/virsh-network.c: In function 'cmdNetworkDHCPLeases':
../../tools/virsh-network.c:1370:12: error: potential null pointer dereference [-Werror=null-dereference]
         ts = *localtime_r(&expirytime_tmp, &ts);
         ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>