Fix a null pointer exception in SVDF

This is due to not checking that `GetVariableInput` returns non-null tensor.

Also fix a potential null pointer exception in `GetVariableInput`.

PiperOrigin-RevId: 385160147
Change-Id: Iadf3f0705b036a9014d27caa5a8bbd91f4c4c401

Author: mihaimaruseac

tensorflow/lite/kernels/kernel_util.cc

@@ -119,6 +119,7 @@ TfLiteStatus GetInputSafe(const TfLiteContext* context, const TfLiteNode* node,
 TfLiteTensor* GetVariableInput(TfLiteContext* context, const TfLiteNode* node,
                                int index) {
   TfLiteTensor* tensor = GetMutableInput(context, node, index);
+  if (tensor == nullptr) return nullptr;
   return tensor->is_variable ? tensor : nullptr;
 }
 

tensorflow/lite/kernels/svdf.cc

@@ -299,6 +299,7 @@ TfLiteStatus Eval(TfLiteContext* context, TfLiteNode* node) {
                     GetTemporarySafe(context, node, /*index=*/0, &scratch));
 
   TfLiteTensor* state = GetVariableInput(context, node, kStateTensor);
+  TF_LITE_ENSURE(context, state != nullptr);
   TfLiteTensor* output;
   TF_LITE_ENSURE_OK(context,
                     GetOutputSafe(context, node, kOutputTensor, &output));