Bug 907288 - [Messages] Recipients.prototype.remove should guard against calls to splice(-1, 1)

https://bugzilla.mozilla.org/show_bug.cgi?id=907288

- If index === -1, return
- Tests:
    - Assert recipients.length correctness
    - Assert call to render

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>