[PATCH] Fix ActiveSupport::Multibyte::Chars#slice for empty strings when starting offset is negative

ActiveSupport::Multibyte::Chars#slice crashes for a negative starting offset if the string is empty.

Steps to reproduce:

$ script/console
Loading development environment (Rails 2.3.8)

rails(test)> 'test'.slice(-1, 1)
=> "t"

rails(test)> ''.slice(-1, 1)
=> nil

rails(test)> 'test'.mb_chars.slice(-1, 1)
=> #<ActiveSupport::Multibyte::Chars:0x103979830 @wrapped_string="t">

rails(test)> ''.mb_chars.slice(-1, 1)
NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.pack
    from /opt/local/lib/ruby/gems/1.8/gems/activesupport-2.3.8/lib/active_support/multibyte/chars.rb:347:in `slice'
    from (irb):4

See the attached patch.