clients: keyboard: fix delete before cursor

When editing a text field in chromium deleting the surrounding text with
backspace fails with `The selection range for surrounding text is invalid`

It seems like `(start - keyboard->surrounding_text) - keyboard->surrounding_cursor` evaluates to -1.

As far as i can tell `start - keyboard->surrounding_text` evaluates to
the index in the surrounding_text char arrays that should be deleted.

Signed-off-by: Aske Bækdal Møller <aske@geanix.com>