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/script.h

@@ -770,12 +770,12 @@ public:
     void Serialize(Stream &s, int nType, int nVersion) const {
         std::vector<unsigned char> compr;
         if (Compress(compr)) {
-            s << CFlatData(&compr[0], &compr[compr.size()]);
+            s << CFlatData(compr);
             return;
         }
         unsigned int nSize = script.size() + nSpecialScripts;
         s << VARINT(nSize);
-        s << CFlatData(&script[0], &script[script.size()]);
+        s << CFlatData(script);
     }
 
     template<typename Stream>
@@ -784,13 +784,13 @@ public:
         s >> VARINT(nSize);
         if (nSize < nSpecialScripts) {
             std::vector<unsigned char> vch(GetSpecialSize(nSize), 0x00);
-            s >> REF(CFlatData(&vch[0], &vch[vch.size()]));
+            s >> REF(CFlatData(vch));
             Decompress(nSize, vch);
             return;
         }
         nSize -= nSpecialScripts;
         script.resize(nSize);
-        s >> REF(CFlatData(&script[0], &script[script.size()]));
+        s >> REF(CFlatData(script));
     }
 };