Fix issues with Function_Params constructors (array and vector args)

- code (on MSVC) was asserting due to trying to dereference invalid
  pointers (dereferencing the end iterator, even if only to get its
  address!).
- when a Function_Params is constructed with an empty vector, you
  can't return the address of the vec.front() -- instead we use
  nullptr for the m_begin and m_end pointers.

include/chaiscript/dispatchkit/function_params.hpp

@@ -30,13 +30,19 @@ namespace chaiscript {
       }
 
       explicit Function_Params(const std::vector<Boxed_Value> &vec)
-        : m_begin(&vec.front()), m_end(&vec.front() + vec.size())
+        : m_begin(vec.empty() ? nullptr : &vec.front()), m_end(vec.empty() ? nullptr : &vec.front() + vec.size())
       {
       }
 
       template<size_t Size>
       constexpr explicit Function_Params(const std::array<Boxed_Value, Size> &a)
-        : m_begin(&*std::begin(a)), m_end(&*std::end(a))
+        : m_begin(&a.front()), m_end(&a.front() + Size)
+      {
+      }
+
+      template<>
+      constexpr explicit Function_Params(const std::array<Boxed_Value, size_t{0}> &a)
+        : m_begin(nullptr), m_end(nullptr)
       {
       }