TFL: Fix Subgraph::context_ initialization.

As more and more methods have been added to context_, having a default null initialization of context_ make it safer.

context_ is a POD-struct so no default initializer. context_() will zero-initialize all the members.

PiperOrigin-RevId: 284514663
Change-Id: I21838fc7c769911db9585088f4094b461bb3518a

tensorflow/lite/core/subgraph.h

@@ -532,7 +532,7 @@ class Subgraph {
   // A pure C data structure used to communicate with the pure C plugin
   // interface. To avoid copying tensor metadata, this is also the definitive
   // structure to store tensors.
-  TfLiteContext context_;
+  TfLiteContext context_ = {};
 
   // A pointer to the external contexts (kTfLiteMaxExternalContexts) array that
   // sits inside the associated TFLite interpreter instance.