Get rid of `helm-multi-occur-buffer-list' global value.
* helm-regexp.el (helm-moccur-init,helm-multi-occur-1, helm-occur): Do it.
* helm-regexp.el (helm-occur-from-isearch): Get rid of buffers global value here too.
* helm-regexp.el (helm-occur-from-isearch): Don't set buffers locally twice.