[libcxx] Fix basic_stringbuf constructor

Summary:
[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.

Reviewers: EricWF, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40707

llvm-svn: 320604