Add better error message to ValidateInstructions (#2365)

Fixes #2364
diff --git a/test/val/val_fixtures.h b/test/val/val_fixtures.h
index 69bc9d8..fd5368b 100644
--- a/test/val/val_fixtures.h
+++ b/test/val/val_fixtures.h
@@ -125,6 +125,13 @@
 template <typename T>
 spv_result_t ValidateBase<T>::ValidateInstructions(spv_target_env env) {
   DestroyDiagnostic();
+  if (binary_ == nullptr) {
+    fprintf(stderr,
+            "ERROR: Attempting to validate a null binary, did you forget to "
+            "call CompileSuccessfully?");
+    fflush(stderr);
+  }
+  assert(binary_ != nullptr);
   return spvValidateWithOptions(ScopedContext(env).context, options_,
                                 get_const_binary(), &diagnostic_);
 }