Fix crash in CoroutineQueue

[mickael-menu]

CoroutineQueue could crash during initialization if the tasks coroutine executes before the object is fully initialized. Moving the tasks property above init should fix the issue.

java.lang.NullPointerException: Attempt to invoke interface method 'kotlinx.coroutines.channels.ChannelIterator kotlinx.coroutines.channels.Channel.iterator()' on a null object reference
    at org.readium.r2.shared.util.CoroutineQueue$1.invokeSuspend(CoroutineQueue.kt:36)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
    at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:108)