bug#18936: ERC: lexical-binding in 24.4 breaks erc-send-pre-hook

[xristos]

>From erc.el, erc-send-pre-hook documentation:

"To change the text that will be sent, set the variable STR which is
used in `erc-send-current-line'."

>From erc.el:5272, function `erc-send-input'

    (let ((str input)
          (erc-insert-this t))
      (setq erc-send-this t)
      (run-hook-with-args 'erc-send-pre-hook input)

STR is dynamically bound.