Allow BufferView indices to be unspecified.

Allow BufferView indices for element array buffers to be unspecified to support some extensions.

Note that this is similar to how invalid array buffers are handled in order to support, for example, sparse morph targets.

tiny_gltf.h

@@ -6147,12 +6147,19 @@ bool TinyGLTF::LoadFromString(Model *model, std::string *err, std::string *warn,
 
         const auto bufferView =
             model->accessors[size_t(primitive.indices)].bufferView;
-        // bufferView could be null(-1) for certain extensions
-        if (bufferView >= 0 && bufferView < model->bufferViews.size()) {
+        if (bufferView < 0) {
+          // skip, bufferView could be null(-1) for certain extensions
+        } else if (size_t(bufferView) >= model->bufferViews.size()) { 
+          if (err) {
+            (*err) += "accessor[" + std::to_string(primitive.indices) +
+                      "] invalid bufferView";
+          }
+          return false;
+        } else {
           model->bufferViews[size_t(bufferView)].target =
               TINYGLTF_TARGET_ELEMENT_ARRAY_BUFFER;
-          // we could optionally check if accessors' bufferView type is Scalar,
-          // as it should be
+          // we could optionally check if accessors' bufferView type is Scalar, as
+          // it should be
         }
       }