exp->common.blk cannot be NULL, nbd_export_delete() is only called (through
a bottom half) from blk_exp_unref() and in turn that can only happen
after blk_exp_add() has asserted exp->blk != NULL.
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>