llmodel: add model wrapper destructor, fix mem leak in golang bindings (#862)

Signed-off-by: Juuso Alasuutari <juuso.alasuutari@gmail.com>

gpt4all-bindings/golang/binding.cpp

@@ -25,6 +25,7 @@ void* load_model(const char *fname, int n_threads) {
         return nullptr;
     }
     if (!llmodel_loadModel(model, fname)) {
+        llmodel_model_destroy(model);
         return nullptr;
     }