network: ignore Broadcast= setting when the address is null

When an address is requested with null address, then broadcast should be
determined after an address aquired that will be assigned to the
interface.

src/network/networkd-address.c

@@ -2091,6 +2091,10 @@ static void address_section_adjust_broadcast(Address *address) {
                 log_warning("%s: broadcast address is set for an IPv4 address with peer address. "
                             "Ignoring Broadcast= setting in the [Address] section from line %u.",
                             address->section->filename, address->section->line);
+        else if (!in4_addr_is_set(&address->in_addr.in))
+                log_warning("%s: broadcast address is set for an IPv4 address with null address. "
+                            "Ignoring Broadcast= setting in the [Address] section from line %u.",
+                            address->section->filename, address->section->line);
         else
                 /* Otherwise, keep the specified broadcast address. */
                 return;