shared/bus-message-util: silence gcc warning

shared/bus-message-util: silence gcc warning

[1/429] Compiling C object src/shared/libsystemd-shared-248.a.p/bus-message-util.c.o
../src/shared/bus-message-util.c: In function ‘bus_message_read_dns_servers’:
../src/shared/bus-message-util.c:165:21: warning: ‘family’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  165 |                 r = in_addr_full_new(family, &a, port, 0, server_name, dns + n);
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/shared/bus-message-util.c:165:21: warning: ‘port’ may be used uninitialized in this function [-Wmaybe-uninitialized]
../src/shared/bus-message-util.c:165:21: warning: ‘server_name’ may be used uninitialized in this function [-Wmaybe-uninitialized]

The warning would be there despite all the asserts in bus_error_setfv() and
sd_bus_error_set(). So let's add an explicit assert.