Fix a segfault in fbson under Mac OS X compiler

Summary:
The problem appears to be caused by a bug in Mac OS X compiler
(http://llvm.org/bugs/show_bug.cgi?id=15337). We need explicitly construct the
base object std::ostream(std::streambuf*) with nullptr. Otherwise, ostream will
try to delete the underlying streambuf* which apparently is undefined in the
Mac OS X compiler.

https://github.com/facebook/rocksdb/issues/525

Test Plan:
unit test in fbson
make all check
document_db_test (on mac)

Reviewers: IslamAbdelRahman, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D34587

third-party/fbson/FbsonStream.h

@@ -62,7 +62,11 @@ class FbsonInBuffer : public std::streambuf {
 class FbsonOutStream : public std::ostream {
  public:
   explicit FbsonOutStream(uint32_t capacity = 1024)
-      : head_(nullptr), size_(0), capacity_(capacity), alloc_(true) {
+      : std::ostream(nullptr),
+        head_(nullptr),
+        size_(0),
+        capacity_(capacity),
+        alloc_(true) {
     if (capacity_ == 0) {
       capacity_ = 1024;
     }
@@ -71,7 +75,11 @@ class FbsonOutStream : public std::ostream {
   }
 
   FbsonOutStream(char* buffer, uint32_t capacity)
-      : head_(buffer), size_(0), capacity_(capacity), alloc_(false) {
+      : std::ostream(nullptr),
+        head_(buffer),
+        size_(0),
+        capacity_(capacity),
+        alloc_(false) {
     assert(buffer && capacity_ > 0);
   }