Fix vector_data failure under debug (#4606)

include/flatbuffers/stl_emulation.h

@@ -51,12 +51,12 @@ inline char string_back(const std::string &value) {
 template <typename T> inline T *vector_data(std::vector<T> &vector) {
   // In some debug environments, operator[] does bounds checking, so &vector[0]
   // can't be used.
-  return &(*vector.begin());
+  return vector.empty() ? nullptr : &vector[0];
 }
 
 template <typename T> inline const T *vector_data(
     const std::vector<T> &vector) {
-  return &(*vector.begin());
+  return vector.empty() ? nullptr : &vector[0];
 }
 
 template <typename T, typename V>