Fix ZeroCopyOutputStream:

Added a destructor that commits the last block of data
if there was no final call to BackUp.

src/ripple/overlay/impl/ZeroCopyStream.h

@@ -152,6 +152,8 @@ public:
     ZeroCopyOutputStream (Streambuf& streambuf,
         std::size_t blockSize);
 
+    ~ZeroCopyOutputStream();
+
     bool
     Next (void** data, int* size) override;
 
@@ -177,6 +179,13 @@ ZeroCopyOutputStream<Streambuf>::ZeroCopyOutputStream(
 {
 }
 
+template <class Streambuf>
+ZeroCopyOutputStream<Streambuf>::~ZeroCopyOutputStream()
+{
+    if (commit_ != 0)
+        streambuf_.commit(commit_);
+}
+
 template <class Streambuf>
 bool
 ZeroCopyOutputStream<Streambuf>::Next(