only process deposits if merkleizer available (#3774)

`m.depositsChain.blocks.len` may change during `startEth1Syncing`. If
that happened, an additional check now ensures that `scratchMerkleizer`
was initialized before attempting to use it.

beacon_chain/eth1/eth1_monitor.nim

@@ -1469,7 +1469,7 @@ proc startEth1Syncing(m: Eth1Monitor, delayBeforeStart: Duration) {.async.} =
         m.terminalBlockHash = some terminalBlockCandidate.hash
         m.terminalBlockNumber = some terminalBlockCandidate.number
 
-    if shouldProcessDeposits:
+    if shouldProcessDeposits and scratchMerkleizer != nil:
       if m.latestEth1BlockNumber <= m.cfg.ETH1_FOLLOW_DISTANCE:
         continue