When loading untrusted glTF files, ideally an error should be returned
if the file is malformed instead of an exception/crash. Add additional
validation for crashes found when running tinygltf under a fuzzer, and
add test cases to confirm:
1. Validate that the primitive indices value is within the
model->accessors bounds before dereferencing.
2. Validate that the accessors bufferView index if valid.
3. Validate that the buffer's index is valid when parsing images.
4. For glb files, validate that the overall length is within the
provided input buffer.
