Fix exception due to missing read lock.

[ghost]

Related to #6551

Fixes the following exception at startup:

Feb-15 16:00:10.554 [SeedNodeMain] ERROR b.c.s.CommonSetup: Uncaught Exception from thread SeedNodeMain 
Feb-15 16:00:10.554 [SeedNodeMain] ERROR b.c.s.CommonSetup: throwableMessage= attempt to unlock read lock, not locked by current thread 
Feb-15 16:00:10.554 [SeedNodeMain] ERROR b.c.s.CommonSetup: throwableClass= class java.lang.IllegalMonitorStateException 
Feb-15 16:00:10.602 [SeedNodeMain] ERROR b.c.s.CommonSetup: Stack trace:
java.lang.IllegalMonitorStateException: attempt to unlock read lock, not locked by current thread
    at java.base/java.util.concurrent.locks.ReentrantReadWriteLock$Sync.unmatchedUnlockException(ReentrantReadWriteLock.java:448)
    at java.base/java.util.concurrent.locks.ReentrantReadWriteLock$Sync.tryReleaseShared(ReentrantReadWriteLock.java:432)
    at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.releaseShared(AbstractQueuedSynchronizer.java:1382)
    at java.base/java.util.concurrent.locks.ReentrantReadWriteLock$ReadLock.unlock(ReentrantReadWriteLock.java:897)
    at bisq.core.dao.burningman.accounting.storage.BurningManAccountingStore.getBlockAtHeight(BurningManAccountingStore.java:102)
    at bisq.core.dao.burningman.accounting.storage.BurningManAccountingStoreService.getBlockAtHeight(BurningManAccountingStoreService.java:85)
    at bisq.core.dao.burningman.accounting.BurningManAccountingService.getBlockAtHeight(BurningManAccountingService.java:148)
    at bisq.core.dao.burningman.accounting.node.full.AccountingFullNode.parseBlock(AccountingFullNode.java:244)
    at bisq.core.dao.burningman.accounting.node.full.AccountingFullNode.lambda$requestBlockRecursively$8(AccountingFullNode.java:226)
    at bisq.core.dao.node.full.RpcService$4.lambda$onSuccess$0(RpcService.java:385)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:834)

[alvasw]

utACK

[alejandrogarcia83]

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