[libc++][C++03] Cherry-pick #100879 (#163370)

Author: philnik777

libcxx/include/__cxx03/sstream

@@ -354,9 +354,15 @@ private:
 
 public:
   // [stringbuf.cons] constructors:
-  _LIBCPP_HIDE_FROM_ABI basic_stringbuf() : __hm_(nullptr), __mode_(ios_base::in | ios_base::out) {}
+  _LIBCPP_HIDE_FROM_ABI basic_stringbuf() : __hm_(nullptr), __mode_(ios_base::in | ios_base::out) {
+    // it is implementation-defined whether we initialize eback() & friends to nullptr, and libc++ doesn't
+    __init_buf_ptrs();
+  }
 
-  _LIBCPP_HIDE_FROM_ABI explicit basic_stringbuf(ios_base::openmode __wch) : __hm_(nullptr), __mode_(__wch) {}
+  _LIBCPP_HIDE_FROM_ABI explicit basic_stringbuf(ios_base::openmode __wch) : __hm_(nullptr), __mode_(__wch) {
+    // it is implementation-defined whether we initialize eback() & friends to nullptr, and libc++ doesn't
+    __init_buf_ptrs();
+  }
 
   _LIBCPP_HIDE_FROM_ABI explicit basic_stringbuf(const string_type& __s,
                                                  ios_base::openmode __wch = ios_base::in | ios_base::out)

libcxx/test/libcxx-03/input.output/string.streams/stringbuf/const_sso_buffer.pass.cpp

@@ -8,8 +8,6 @@
 
 // <sstream>
 
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
 // How the constructors of basic_stringbuf initialize the buffer pointers is
 // not specified. For some constructors it's implementation defined whether the
 // pointers are set to nullptr. Libc++'s implementation directly uses the SSO

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.cons/default.pass.cpp

@@ -15,8 +15,6 @@
 // basic_stringbuf() : basic_stringbuf(ios_base::in | ios_base::out) {}               // C++20
 // explicit basic_stringbuf(ios_base::openmode which);                                // C++20
 
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
 #include <sstream>
 #include <cassert>