fix bug where reorg occurs precisely at known chaintip height

Author: arik-so

xcode/DirectBindingsApp/DirectBindingsApp/app-batteries/BlockchainObserver.swift

@@ -193,15 +193,14 @@ class BlockchainObserver {
         }
 
         // create an array of the new blocks
-        // the variable is called anchored because there should be an overlap of one
         var addedBlocks = [RPCBlockDetails]()
         for addedBlockHeight in (knownChaintip.height + 1)...currentChaintipHeight {
             let addedBlockHash = try await self.getBlockHash(height: addedBlockHeight)
             let addedBlock = try await self.getBlock(hash: addedBlockHash)
             addedBlocks.append(addedBlock)
         }
 
-        while addedBlocks.first!.previousblockhash != self.connectedBlocks.last!.hash {
+        while addedBlocks.isEmpty || addedBlocks.first!.previousblockhash != self.connectedBlocks.last!.hash {
             // we must keep popping until it matches
             let trimmedLocalTip = try await self.disconnectBlock()
             let reorgedBlockHash = try await self.getBlockHash(height: trimmedLocalTip.height)