Expand filename param to helm-find-files-1.

Repro:

1. ./emacs-helm.sh
2. eval
        (helm-find-files-1 "~/")
3. type "/sudo:"
4. see:

        Debugger entered--Lisp error: (args-out-of-range #<buffer *Helm Find Files*> 0 1)
          buffer-substring-no-properties(1 0)
          helm-get-selection()
          helm-ff-move-to-first-real-candidate()
          run-hooks(helm-after-update-hook)
          helm-log-run-hook(helm-after-update-hook)
          #[0 "\300\203^[^@\303\242\203^[^@\302\242\203^[^@\306\307\300\"\210\310\300\301\"\210\202J^@\303\242\203'^@\311\312!\210\202J^@\311 \210\313\314\301\"\20$
          funcall(#[0 "\300\203^[^@\303\242\203^[^@\302\242\203^[^@\306\307\300\"\210\310\300\301\"\210\202J^@\303\242\203'^@\311\312!\210\202J^@\311 \210\313\314\$
          helm-update()