Strengthen requirements to call G1HeapVerifier::verify(VerifyOption)

G1HeapVerifier::verify() seems to miss a return:

void G1HeapVerifier::verify(VerifyOption vo) {
  if (!SafepointSynchronize::is_at_safepoint()) {
    log_info(gc, verify)("Skipping verification. Not at safepoint.");
    // here?
  }
  assert(Thread::current()->is_VM_thread(),
         "Expected to be executed serially by the VM thread at this point");

Found by [~stefank] during discussion of JDK-8256641.

This seems to be the case since initial import of that file.