nbd: avoid uninitialized warnings

==15815== Thread 1:
==15815== Syscall param socketcall.sendto(msg) points to uninitialised byte(s)
==15815==    at 0x65AD5CB: send (send.c:31)
==15815==    by 0x37F84B: nbd_wr_sync (nbd.c:145)
==15815==    by 0x37F94B: write_sync (nbd.c:186)
==15815==    by 0x380FA9: nbd_send_request (nbd.c:681)
==15815==    by 0x1C4A2D: nbd_teardown_connection (nbd-client.c:337)
==15815==    by 0x1C4AD8: nbd_client_session_close (nbd-client.c:354)
==15815==    by 0x1ED2D8: close_socketpair (spicebd.c:132)
==15815==    by 0x1EE265: spice_close (spicebd.c:457)
==15815==    by 0x1ACBF6: bdrv_close (block.c:1519)
==15815==    by 0x1AD804: bdrv_delete (block.c:1772)
==15815==    by 0x1B4136: bdrv_unref (block.c:4476)
==15815==    by 0x1ACCE0: bdrv_close (block.c:1541)
==15815==  Address 0x7feffef98 is on thread 1's stack

Signed-off-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
