[Patch] Fix seek to beg after grow bug

[Paolo Carlini]

Paolo Carlini wrote:

> No, this cannot be ok.
> Look at the testcase: _M_out_cur == _M_out_beg in that case and 
> _M_out_end,
> which points one-past-end, gives the current string length, as 
> _M_out_end -
> _M_out_cur which is _bigger_ than _M_string.size().

Sorry, typo, I really meant: ... as _M_out_end - _M_out_beg...
(but in this specific case it's the same, since _M_out_beg == _M_out_cur)

Paolo.