reset error return trace index

codegen.cpp

    if (instruction->operand != nullptr &&
        instruction->operand->value->type->id != ZigTypeIdErrorSet &&
        instruction->operand->value->type->id != ZigTypeIdErrorUnion)
    {
        // Reset error return trace index.
        bool is_llvm_alloca = false;
        LLVMValueRef err_ret_trace_ptr = get_cur_err_ret_trace_val(g, instruction->base.base.scope, &is_llvm_alloca);
        if (err_ret_trace_ptr != nullptr) {
            size_t index_field_index = g->stack_trace_type->data.structure.fields[0]->gen_index;
            LLVMValueRef index_field_ptr = LLVMBuildStructGEP(g->builder, err_ret_trace_ptr, (unsigned)index_field_index, "");
            LLVMValueRef zero = LLVMConstNull(g->builtin_types.entry_usize->llvm_type);
            gen_store_untyped(g, zero, index_field_ptr, 0, false);
        }
    }