Guard nil asset.contentURL in ETCoreMLModel initWithAsset:

Differential Revision: D108334793

Pull Request resolved: pytorch#20338

Author: KushalAdhvaryu

backends/apple/coreml/runtime/delegate/ETCoreMLModel.mm

@@ -194,7 +194,15 @@ - (nullable instancetype)initWithAsset:(ETCoreMLAsset *)asset
         return nil;
     }
 
-    MLModel *mlModel = [MLModel modelWithContentsOfURL:asset.contentURL
+    NSURL *contentURL = asset.contentURL;
+    if (contentURL == nil) {
+        ETCoreMLLogErrorAndSetNSError(error,
+                                      ETCoreMLErrorCorruptedModel,
+                                      "asset.contentURL is nil");
+        return nil;
+    }
+
+    MLModel *mlModel = [MLModel modelWithContentsOfURL:contentURL
                                          configuration:configuration
                                                  error:error];
     if (!mlModel) {