Fixed FlatBufferBuilder::Required not checking vtable size.

As reported in: https://github.com/google/flatbuffers/issues/4846
Tested: on Linux.

Change-Id: Id5e82dc5a46a681119cfe5a15415d522aac0e1f2

include/flatbuffers/flatbuffers.h

@@ -1038,14 +1038,7 @@ class FlatBufferBuilder {
 
   // This checks a required field has been set in a given table that has
   // just been constructed.
-  template<typename T> void Required(Offset<T> table, voffset_t field) {
-    auto table_ptr = buf_.data_at(table.o);
-    auto vtable_ptr = table_ptr - ReadScalar<soffset_t>(table_ptr);
-    bool ok = ReadScalar<voffset_t>(vtable_ptr + field) != 0;
-    // If this fails, the caller will show what field needs to be set.
-    FLATBUFFERS_ASSERT(ok);
-    (void)ok;
-  }
+  template<typename T> void Required(Offset<T> table, voffset_t field);
 
   uoffset_t StartStruct(size_t alignment) {
     Align(alignment);
@@ -2117,6 +2110,15 @@ class Table {
   uint8_t data_[1];
 };
 
+template<typename T> void FlatBufferBuilder::Required(Offset<T> table,
+                                                      voffset_t field) {
+  auto table_ptr = reinterpret_cast<const Table *>(buf_.data_at(table.o));
+  bool ok = table_ptr->GetOptionalFieldOffset(field) != 0;
+  // If this fails, the caller will show what field needs to be set.
+  FLATBUFFERS_ASSERT(ok);
+  (void)ok;
+}
+
 /// @brief This can compute the start of a FlatBuffer from a root pointer, i.e.
 /// it is the opposite transformation of GetRoot().
 /// This may be useful if you want to pass on a root and have the recipient