virsh: fix entering interactive session

virsh: fix entering interactive session

When entering an interactive session, that is, without a command
specified:

  virsh --connect $URI

virsh currently segfaults because it tries to access
ctl->cmd->def->handler and the ctl->cmd is NULL.

Fix by checking if ctl->cmd is not NULL before doing further checks.

Fixes: b489eb8d6b52d4183754a5d1da55006148a331e4
Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>