[Patch] Remove a dead seekoff from _M_underflow

[Paolo Carlini]

Hi,

consider this code from the current _M_underflow:

 if (this->_M_in_cur < this->_M_in_end)
   {
      __ret = traits_type::to_int_type(*this->_M_in_cur);
      if (__bump)
        _M_move_in_cur(1);
      return __ret;
   }
 
 // Sync internal and external buffers.
 // NB: __testget -> __testput as _M_buf_unified here.
 if (this->_M_in_cur > this->_M_in_beg)
   {
     if (__testout)
       _M_overflow();
     else if (this->_M_in_cur != _M_filepos)
       _M_file.seekoff(this->_M_in_cur - _M_filepos, ios_base::cur,
                       ios_base::in);
   }

When !__testout for sure _M_out_cur_move is never called and this
implies that _M_in_end is stuck at the value set by the last
_M_set_buffer call. Since in the code above __testin is true,
_M_in_end == _M_filepos (see _M_set_buffer) and therefore, in the
second block above always _M_in_cur == _M_filepos.

I also take the occasion to rewrite the resulting simplified 'if'
in terms of _M_out_* pointers (equivalent but more clear, IMO) and
remove a redundant 'if' not shown above: everywhere else
_M_destroy_pback is called without checking for _M_pback_init, this
is only a remnant from a previous more convoluted 'if' block.

Tested x86-linux, also put an abort() in the original code in place
of the removed 'if' branch and ran the testsuite.

Paolo.

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