[in_app_purchase_storekit] Fixes manual invocation of finishTransaction() triggering fatal crash

[LouiseHsu]

Fixes flutter/flutter#154763

From the Apple docs:
https://developer.apple.com/documentation/storekit/skpaymentqueue/1506003-finishtransaction
If you call finishTransaction(_:) on a transaction that is in the [SKPaymentTransactionState.purchasing](https://developer.apple.com/documentation/storekit/skpaymenttransactionstate/purchasing) state, StoreKit raises an exception.

For some reason even though the old Obj-C implementation didn't have this check, it didn't crash. This adds an explicit check for the purchasing state.

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