network: fix conditional jump depends on uninitialised value(s)

network: fix conditional jump depends on uninitialised value(s)

When address is in IPv4, the remaining buffer in in_addr_union may
not be initialized.

Fixes the following valgrind warning:
```
==13169== Conditional jump or move depends on uninitialised value(s)
==13169==    at 0x137FF6: UnknownInlinedFun (networkd-ndisc.c:77)
==13169==    by 0x137FF6: UnknownInlinedFun (networkd-ndisc.c:580)
==13169==    by 0x137FF6: ndisc_handler.lto_priv.83 (networkd-ndisc.c:597)
==13169==    by 0x11BE23: UnknownInlinedFun (sd-ndisc.c:201)
==13169==    by 0x11BE23: ndisc_recv.lto_priv.174 (sd-ndisc.c:254)
==13169==    by 0x4AA18CF: source_dispatch (sd-event.c:2821)
==13169==    by 0x4AA1BC2: sd_event_dispatch (sd-event.c:3234)
==13169==    by 0x4AA1D88: sd_event_run (sd-event.c:3291)
==13169==    by 0x4AA1FAB: sd_event_loop (sd-event.c:3313)
==13169==    by 0x117401: UnknownInlinedFun (networkd.c:113)
==13169==    by 0x117401: main (networkd.c:120)
==13169==  Uninitialised value was created by a stack allocation
==13169==    at 0x1753C8: manager_rtnl_process_address (networkd-manager.c:479)
```