[retry] don't start a new call attempt if the timer was cancelled (#35729)

Fixes #34844.

Closes #35729

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35729 from markdroth:retry_fix 899cb370ba
PiperOrigin-RevId: 603060248

src/core/ext/filters/client_channel/retry_filter_legacy_call_data.cc

@@ -2020,8 +2020,10 @@ void RetryFilter::LegacyCallData::OnRetryTimer() {
 void RetryFilter::LegacyCallData::OnRetryTimerLocked(
     void* arg, grpc_error_handle /*error*/) {
   auto* calld = static_cast<RetryFilter::LegacyCallData*>(arg);
+  if (calld->retry_timer_handle_.has_value()) {
     calld->retry_timer_handle_.reset();
     calld->CreateCallAttempt(/*is_transparent_retry=*/false);
+  }
   GRPC_CALL_STACK_UNREF(calld->owning_call_, "OnRetryTimer");
 }