streams: Add data() method to CDataStream

Analogous to c++11 vector data().

(cherry picked from commit af4c44ce59aa643b3e03eaa121ec075f524227a5)

src/streams.h

@@ -177,6 +177,8 @@ public:
     void clear()                                     { vch.clear(); nReadPos = 0; }
     iterator insert(iterator it, const char& x=char()) { return vch.insert(it, x); }
     void insert(iterator it, size_type n, const char& x) { vch.insert(it, n, x); }
+    value_type* data()                               { return vch.data() + nReadPos; }
+    const value_type* data() const                   { return vch.data() + nReadPos; }
 
     void insert(iterator it, std::vector<char>::const_iterator first, std::vector<char>::const_iterator last)
     {