Check for missing chain instructions (#1510)

Summary:
Pull Request resolved: #1510

Don't use the `instructions` field of `Chain` unless it's present.

Reviewed By: larryliu0820

Differential Revision: D52451741

fbshipit-source-id: d1a09f30d3b46196bb756244c1e8867ad4be483c

Author: dbort

runtime/executor/method.cpp

@@ -598,17 +598,23 @@ Error Method::init(executorch_flatbuffer::ExecutionPlan* s_plan) {
     int32_t num_instructions_missing_op = 0;
     for (size_t i = 0; i < n_chains_; ++i) {
       auto s_chain = chains->Get(i);
-      auto num_instructions = s_chain->instructions()->size();
+      auto s_instructions = s_chain->instructions();
+      ET_CHECK_OR_RETURN_ERROR(
+          s_instructions != nullptr,
+          InvalidProgram,
+          "Missing instructions in chain %zu",
+          i);
+      auto num_instructions = s_instructions->size();
       auto chain_instruction_kernels = ET_ALLOCATE_LIST_OR_RETURN_ERROR(
           method_allocator, OpFunction, num_instructions);
       auto chain_instruction_arg_lists = ET_ALLOCATE_LIST_OR_RETURN_ERROR(
           method_allocator, InstructionArgs, num_instructions);
 
       // Set up the argument lists ahead of time and store pointers to them to
       // use when the instructions are called
-      for (size_t instr_idx = 0; instr_idx < s_chain->instructions()->size();
+      for (size_t instr_idx = 0; instr_idx < s_instructions->size();
            ++instr_idx) {
-        const auto instruction = s_chain->instructions()->Get(instr_idx);
+        const auto instruction = s_instructions->Get(instr_idx);
         switch (instruction->instr_args_type()) {
           case executorch_flatbuffer::InstructionArguments::KernelCall: {
             const auto arg_idxs =