[NFC][libc++] Fix whitespace in sstream

[Mordante]

LGTM, thanks. I'll commit it on your behalf.

libcxx/include/sstream

basic_stringbuf<_CharT, _Traits, _Allocator>::str() const {
     return string_type(__str_.get_allocator());
 }
 #endif // _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_BUILDING_LIBRARY)
 
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI void basic_stringbuf<_CharT, _Traits, _Allocator>::__init_buf_ptrs() {
     __hm_ = nullptr;
     char_type* __data = const_cast<char_type*>(__str_.data());
-        typename string_type::size_type __sz = __str_.size();
     if (__mode_ & ios_base::in) {
         __hm_ = __data + __sz;
         this->setg(__data, __data, __hm_);
     }
     if (__mode_ & ios_base::out) {
         __hm_ = __data + __sz;
         __str_.resize(__str_.capacity());
         this->setp(__data, __data + __str_.size());

 }
 
 template <class _CharT, class _Traits, class _Allocator>
 typename basic_stringbuf<_CharT, _Traits, _Allocator>::int_type
 basic_stringbuf<_CharT, _Traits, _Allocator>::overflow(int_type __c)
 {
     if (!traits_type::eq_int_type(__c, traits_type::eof()))
     {
-        ptrdiff_t __ninp = this->gptr()  - this->eback();
+        ptrdiff_t __ninp = this->gptr() - this->eback();
         if (this->pptr() == this->epptr())
         {
             if (!(__mode_ & ios_base::out))
                 return traits_type::eof();
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
             try
             {
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
-                ptrdiff_t __nout = this->pptr()  - this->pbase();
+                ptrdiff_t __nout = this->pptr() - this->pbase();
                 ptrdiff_t __hm = __hm_ - this->pbase();
                 __str_.push_back(char_type());
                 __str_.resize(__str_.capacity());
                 char_type* __p = const_cast<char_type*>(__str_.data());
                 this->setp(__p, __p + __str_.size());
                 this->__pbump(__nout);
                 __hm_ = this->pbase() + __hm;
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS