[tflite] Test for `kTfLiteOptionalTensor` in `GetInput`.

`GetInput`, `GetVariableInput` and `GetOutput` all fail to check for the case where `node->inputs->data[index]` is the special `kTfLiteOptionalTensor` value (-1) which then causes `context->tensors[node->inputs->data[index]]` to read from invalid memory location.

This fix makes `GetInput` and related return `nullptr` in those cases, asking the caller to check for `nullptr`. This is better than having `GetOptionalInputTensor` and `GetOptionalOutputTensor` (does not exist but could be added) as using the patched `GetInput` in error would be caught by a sanitizer test in the default optimized build (due to the `-fsanitize=null` option).

PiperOrigin-RevId: 332512190
Change-Id: Iabca54da2f2de02b6ece3c38b54f76d4277d689e

tensorflow/lite/kernels/kernel_util.h

@@ -30,27 +30,49 @@ inline int SizeOfDimension(const TfLiteTensor* t, int dim) {
 }
 inline const TfLiteTensor* GetInput(const TfLiteContext* context,
                                     const TfLiteNode* node, int index) {
-  return &context->tensors[node->inputs->data[index]];
+  const int tensor_index = node->inputs->data[index];
+  if (tensor_index < 0) {
+    return nullptr;
+  }
+  return &context->tensors[tensor_index];
 }
 // Note: You must check if result is not null:
 // TfLiteTensor* my_tensor = GetVariableInput(context, node, kMyTensorIdx);
 // TF_LITE_ENSURE(context, my_tensor != nullptr);
 inline TfLiteTensor* GetVariableInput(TfLiteContext* context,
                                       const TfLiteNode* node, int index) {
-  TfLiteTensor* tensor = &context->tensors[node->inputs->data[index]];
+  const int tensor_index = node->inputs->data[index];
+  if (tensor_index < 0) {
+    return nullptr;
+  }
+  TfLiteTensor* tensor = &context->tensors[tensor_index];
+>>>>>>> d8f8236c29 ([tflite] Test for `kTfLiteOptionalTensor` in `GetInput`.)
   return (tensor->is_variable) ? tensor : nullptr;
 }
 inline TfLiteTensor* GetOutput(TfLiteContext* context, const TfLiteNode* node,
                                int index) {
-  return &context->tensors[node->outputs->data[index]];
+  const int tensor_index = node->outputs->data[index];
+  if (tensor_index < 0) {
+    return nullptr;
+  }
+  return &context->tensors[tensor_index];
 }
 inline TfLiteTensor* GetTemporary(TfLiteContext* context,
                                   const TfLiteNode* node, int index) {
-  return &context->tensors[node->temporaries->data[index]];
+  const int tensor_index = node->temporaries->data[index];
+  if (tensor_index < 0) {
+    return nullptr;
+  }
+  return &context->tensors[tensor_index];
 }
+
 inline const TfLiteTensor* GetIntermediates(TfLiteContext* context,
                                             const TfLiteNode* node, int index) {
-  return &context->tensors[node->intermediates->data[index]];
+  const int tensor_index = node->intermediates->data[index];
+  if (tensor_index < 0) {
+    return nullptr;
+  }
+  return &context->tensors[tensor_index];
 }
 inline int NumInputs(const TfLiteNode* node) { return node->inputs->size; }
 inline int NumOutputs(const TfLiteNode* node) { return node->outputs->size; }