le_periodic_sync_manager: Check if established sync was pending

That check prevents from executing operations on periodic_sync which
was not started yet. That is possible when there are two requests for
the same broadcaster, typical on pair of buds.

Fail scenario with inactive broadcaster:
1. HandleStartSyncRequest with periodic_sync A
2. OnStartSyncTimeout inform upper layer about A failure, cancel and
remove A
3. HandleStartSyncRequest with periodic_sync B (the same broadcaster)
4. HandleLePeriodicAdvertisingSyncEstablished with error 68 because of
canceling A, informing upper layer about B failure because A is already
removed, removing B.

Bug: 354890418
Test: atest PeriodicSyncManagerTest
Flag: Exempt, trivial fix covered with unit test
Change-Id: I67f4949e2cdd02c5e9757515393f6ce43ca627db