nbd: fix nbd_server_stop crash when no server was running

This failed on the new assertion of qemu_set_fd_handler2:

qemu-system-x86_64: /home/pbonzini/work/upstream/qemu/iohandler.c:60: qemu_set_fd_handler2: Assertion `fd >= 0' failed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
