Added VectorLength helper function that works on nullptr.

Change-Id: Ie62096f7337a476bee7a6d46d652e594fb3124d2
Tested: on Linux.
Bug: 18201051

include/flatbuffers/flatbuffers.h

@@ -296,6 +296,12 @@ template<typename T> class Vector {
   uoffset_t length_;
 };
 
+// Convenient helper function to get the length of any vector, regardless
+// of wether it is null or not (the field is not set).
+template<typename T> static inline size_t VectorLength(const Vector<T> *v) {
+  return v ? v->Length() : 0;
+}
+
 struct String : public Vector<char> {
   const char *c_str() const { return reinterpret_cast<const char *>(Data()); }
 };

tests/test.cpp

@@ -137,6 +137,7 @@ void AccessFlatBufferTest(const std::string &flatbuf) {
   TEST_EQ(pos->test3().b(), 20);
 
   auto inventory = monster->inventory();
+  TEST_EQ(VectorLength(inventory), 10);  // Works even if inventory is null.
   TEST_NOTNULL(inventory);
   unsigned char inv_data[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
   for (auto it = inventory->begin(); it != inventory->end(); ++it)