[libcxx] Fix basic_stringbuf constructor
The C++ Standard [stringbuf.cons]p1 defines the effects of the basic_stringbuf
constructor that takes ios_base::openmode as follows:
Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf(), and initializing mode with which. Postconditions: str() == "".
The default constructor of basic_streambuf shall initialize all its
pointer member objects to null pointers [streambuf.cons]p1.
Currently libc++ calls "str(string_type());" in the aforementioned constructor
setting basic_streambuf's pointers to a non-null value.
This patch removes the call (note that the postcondition str() == ""
remains valid because __str_ is default-initialized) and adds a test checking
that the basic_streambuf's pointers are null after construction.
Thanks Mikhail Maltsev for the patch.