Avoid undefined behavior using CFlatData in CScript serialization

`&vch[vch.size()]` and even `&vch[0]` on vectors can cause assertion
errors with VC in debug mode. This is the problem mentioned in #4239.
The deeper problem with this is that we rely on undefined behavior.

- Add `begin_ptr` and `end_ptr` functions that get the beginning and end
  pointer of vector in a reliable way that copes with empty vectors and
  doesn't reference outside the vector
(see https://stackoverflow.com/questions/1339470/how-to-get-the-address-of-the-stdvector-buffer-start-most-elegantly/1339767#1339767).
- Add a convenience constructor to CFlatData that wraps a vector.

I added `begin_ptr` and `end_ptr` as separate functions as I imagine
they will be useful in more places.

src/serialize.h

@@ -37,6 +37,34 @@ inline T& REF(const T& val)
     return const_cast<T&>(val);
 }
 
+/** Get begin pointer of vector (non-const version).
+ * @note These functions avoid the undefined case of indexing into an empty
+ * vector, as well as that of indexing after the end of the vector.
+ */
+template <class T, class TAl>
+inline T* begin_ptr(std::vector<T,TAl>& v)
+{
+    return v.empty() ? NULL : &v[0];
+}
+/** Get begin pointer of vector (const version) */
+template <class T, class TAl>
+inline const T* begin_ptr(const std::vector<T,TAl>& v)
+{
+    return v.empty() ? NULL : &v[0];
+}
+/** Get end pointer of vector (non-const version) */
+template <class T, class TAl>
+inline T* end_ptr(std::vector<T,TAl>& v)
+{
+    return v.empty() ? NULL : (&v[0] + v.size());
+}
+/** Get end pointer of vector (const version) */
+template <class T, class TAl>
+inline const T* end_ptr(const std::vector<T,TAl>& v)
+{
+    return v.empty() ? NULL : (&v[0] + v.size());
+}
+
 /////////////////////////////////////////////////////////////////
 //
 // Templates for serializing to anything that looks like a stream,
@@ -318,6 +346,12 @@ protected:
     char* pend;
 public:
     CFlatData(void* pbeginIn, void* pendIn) : pbegin((char*)pbeginIn), pend((char*)pendIn) { }
+    template <class T, class TAl>
+    explicit CFlatData(std::vector<T,TAl> &v)
+    {
+        pbegin = (char*)begin_ptr(v);
+        pend = (char*)end_ptr(v);
+    }
     char* begin() { return pbegin; }
     const char* begin() const { return pbegin; }
     char* end() { return pend; }